Light-emitting element, light-emitting device, electronic device, and lighting device

ABSTRACT

A light-emitting element is provided. The light-emitting element includes first and second electrodes and an EL layer therebetween. The EL layer includes a light-emitting layer containing first and second substances. The amount of the first substance is larger than that of the second substance. The second substance emits light. Average transition dipole moments of the second substance are divided into three components in x-, y-, and z-directions which are orthogonal to each other. Components parallel to the first or second electrode are assumed to be the components in the x- and y-directions, and a component perpendicular to the first or second electrode is assumed to be the component in the z-direction. The proportion of the component in the z-direction is represented by a, which is less than or equal to 0.2.

This application is a continuation of U.S. application Ser. No.15/597,339, filed on May 17, 2017 (now U.S. Pat. No. 9,985,234 issuedMay 29, 2018) which is incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a light-emittingelement, a display module, a lighting module, a display device, alight-emitting device, an electronic device, and a lighting device. Notethat one embodiment of the present invention is not limited to the abovetechnical field. The technical field of one embodiment of the inventiondisclosed in this specification and the like relates to an object, amethod, or a manufacturing method. One embodiment of the presentinvention relates to a process, a machine, manufacture, or a compositionof matter. Specifically, examples of the technical field of oneembodiment of the present invention disclosed in this specificationinclude a semiconductor device, a display device, a liquid crystaldisplay device, a light-emitting device, a lighting device, a powerstorage device, a memory device, a method for driving any of them, and amethod for manufacturing any of them.

2. Description of the Related Art

As lighting devices or display devices, display devices includinglight-emitting elements (organic EL elements) in which organic compoundsor organometallic complexes are used as light-emitting substances havebeen developed because of their potential for thinness, lightness,high-speed response to input signals, low power consumption, and thelike.

In an organic EL element, voltage application between electrodes betweenwhich a light-emitting layer is provided causes recombination ofelectrons and holes injected from the electrodes, which brings alight-emitting substance into an excited state, and the return from theexcited state to the ground state is accompanied by light emission.Since the spectrum of light emitted from the light-emitting substancedepends on the light-emitting substance, the use of different types oflight-emitting substances makes it possible to obtain light-emittingelements which exhibit various colors.

Although displays or lighting devices including light-emitting elementsare suitably used for a variety of electronic devices, their performancehas plenty of room to improve. For example, the current efficiency of alight-emitting element is preferably as high as possible. However, theinternal quantum efficiency of a light-emitting element for bothfluorescence and phosphorescence is going to reach the theoreticallimit; in particular, by utilizing a triplet excited state, somelight-emitting elements that emit fluorescence have achieved an internalquantum efficiency of 25%, which is a theoretical limit, or higher.

In order to enhance the current efficiency of a light-emitting elementwhose internal quantum efficiency has been improved, the lightextraction efficiency of the element needs to be improved.

Patent Document 1 discloses a light-emitting element whose lightextraction efficiency is improved by depositing light-emittingsubstances such that their orientations are aligned with each other tocontrol the direction of light emission.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2012-129509

Non-Patent Document

-   [Non-Patent Document 1]-   D. Yokoyama, Journal of Materials Chemistry, 2011, 21, 19187    [Non-Patent Document 2]-   P. Liehm et al., Applied Physics Letters, 101, 253304 (2012)

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide anovel light-emitting element. Another object is to provide a novellight-emitting element with high emission efficiency.

An object of another embodiment of the present invention is to provide adisplay module, a lighting module, a light-emitting device, a displaydevice, an electronic device, and a lighting device each having lowpower consumption.

It is only necessary that at least one of the above objects be achievedin one embodiment of the present invention. Note that the description ofthese objects does not disturb the existence of other objects. In oneembodiment of the present invention, there is no need to achieve all theobjects. Other objects will be apparent from and can be derived from thedescription of the specification, the drawings, the claims, and thelike.

One embodiment of the present invention is a light-emitting elementincluding a first electrode, a second electrode, and an EL layer betweenthe first electrode and the second electrode. The EL layer includes alight-emitting layer. The light-emitting layer contains a firstsubstance and a second substance. The amount of the first substance islarger than the amount of the second substance in the light-emittinglayer. The second substance emits light. The value of a parameter a ofthe light-emitting element is less than or equal to 0.2; the parameter ais a ratio of light emission from a perpendicular component in az-direction to the whole light emission when light emission from averagetransition dipoles of the second substance in the light-emitting layeris divided into light emission from three components of transitiondipoles in an x-direction, a y-direction, and the z-direction which areorthogonal to each other (note that the x-direction and the y-directionare defined as directions parallel to the first electrode or the secondelectrode while the z-direction is defined as a direction perpendicularto the first electrode or the second electrode).

Another embodiment of the present invention is a light-emitting elementhaving the above structure in which the second substance is aphosphorescent substance.

Another embodiment of the present invention is a light-emitting elementhaving the above structure in which the second substance is an iridiumcomplex.

Another embodiment of the present invention is a light-emitting elementhaving the above structure in which the external quantum efficiency ishigher than or equal to 25%.

Another embodiment of the present invention is a light-emitting elementhaving the above structure in which the light-emitting layer furthercontains a third substance and the first substance and the thirdsubstance form an exciplex.

Another embodiment of the present invention is a light-emitting elementhaving the above structure in which the second substance is afluorescent substance.

Another embodiment of the present invention is a light-emitting elementhaving the above structure in which the second substance is a substancehaving a condensed aromatic hydrocarbon skeleton.

Another embodiment of the present invention is a light-emitting elementhaving the above structure in which the external quantum efficiency ishigher than or equal to 7.5%.

Another embodiment of the present invention is a light-emitting elementhaving the above structure in which the external quantum efficiency ishigher than or equal to 10%.

Another embodiment of the present invention is a light-emitting elementhaving the above structure in which light emission from thelight-emitting element includes a delayed fluorescent component.

Another embodiment of the present invention is a light-emitting elementhaving the above structure in which a is greater than or equal to 0 andless than or equal to 0.2.

Another embodiment of the present invention is a light-emitting deviceincluding the above-described light-emitting element and at least one ofa transistor and a substrate.

Another embodiment of the present invention is an electronic deviceincluding the above-described light-emitting device and at least one ofa sensor, an operation button, a speaker, and a microphone.

Another embodiment of the present invention is a lighting deviceincluding the above-described light-emitting device and a housing.

Note that a light-emitting device in this specification includes, in itscategory, an image display device including a light-emitting element. Alight-emitting device may be included in a module in which alight-emitting element is provided with a connector such as ananisotropic conductive film or a tape carrier package (TCP), a module inwhich a printed wiring board is provided at the end of a TCP, and amodule in which an integrated circuit (IC) is directly mounted on alight-emitting element by a chip on glass (COG) method. Thelight-emitting device may also be included in lighting equipment.

In one embodiment of the present invention, a novel light-emittingelement can be provided. Also, a novel light-emitting element with highemission efficiency can be provided.

Another embodiment of the present invention can provide a displaymodule, a lighting module, a light-emitting device, a display device, anelectronic device, and a lighting device each having low powerconsumption.

It is only necessary that at least one of the above effects be achievedin one embodiment of the present invention. Note that the description ofthese effects does not disturb the existence of other effects. Oneembodiment of the present invention does not necessarily achieve all theeffects listed above. Other effects will be apparent from and can bederived from the description of the specification, the drawings, theclaims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing how to calculate the external quantumefficiency of a fluorescent light-emitting element.

FIG. 2 shows a variation in emission intensity depending on thedirection of a transition dipole and the observation angle.

FIGS. 3A to 3C are each a conceptual diagram of a light-emittingelement.

FIGS. 4A to 4D illustrate an example of a method for manufacturing alight-emitting element.

FIG. 5 illustrates an example of a manufacturing apparatus of alight-emitting element.

FIGS. 6A and 6B are conceptual diagrams of an active matrixlight-emitting device.

FIGS. 7A and 7B are conceptual diagrams each illustrating an activematrix light-emitting device.

FIG. 8 is a conceptual diagram of an active matrix light-emittingdevice.

FIGS. 9A and 9B are conceptual diagrams of a passive matrixlight-emitting device.

FIGS. 10A and 10B illustrate a lighting device.

FIGS. 11A to 11D each illustrate an electronic device.

FIG. 12 illustrates a light source device.

FIG. 13 illustrates a lighting device.

FIG. 14 illustrates a lighting device.

FIG. 15 illustrates car-mounted display devices and lighting devices.

FIGS. 16A to 16C illustrate an electronic device.

FIGS. 17A to 17C illustrate an electronic device.

FIG. 18 is a conceptual diagram of a light-emitting element formeasurement.

FIG. 19 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 1.

FIG. 20 shows a method for measuring the angle dependence of theemission spectrum.

FIG. 21 is a graph showing the measured and calculated integratedintensity of the EL emission spectrum depending on the angle (θ) of adetector of Light-emitting Element 1-1.

FIG. 22 is a 2D contour map showing the measured angle dependence of theEL emission spectrum of Light-emitting Element 1-1.

FIG. 23 is a 2D contour map obtained by calculation of Light-emittingElement 1-1.

FIG. 24 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 2.

FIG. 25 is a graph showing the measured and calculated integratedintensity of the EL emission spectrum depending on the angle (θ) of thedetector of Light-emitting Element 2-1.

FIG. 26 is a 2D contour map showing the measured angle dependence of theEL emission spectrum of Light-emitting Element 2-1.

FIG. 27 is a 2D contour map obtained by calculation of Light-emittingElement 2-1.

FIG. 28 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 3.

FIG. 29 is a graph showing the measured and calculated integratedintensity of the EL emission spectrum depending on the angle (θ) of thedetector of Light-emitting Element 3-1.

FIG. 30 is a 2D contour map showing the measured angle dependence of theEL emission spectrum of Light-emitting Element 3-1.

FIG. 31 is a 2D contour map obtained by calculation of Light-emittingElement 3-1.

FIG. 32 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 4.

FIG. 33 is a graph showing the measured and calculated integratedintensity of the EL emission spectrum depending on the angle (θ) of thedetector of Light-emitting Element 4-1.

FIG. 34 is a 2D contour map showing the measured angle dependence of theEL emission spectrum of Light-emitting Element 4-1.

FIG. 35 is a 2D contour map obtained by calculation of Light-emittingElement 4-1.

FIG. 36 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 5.

FIG. 37 is a graph showing the measured and calculated integratedintensity of the EL emission spectrum depending on the angle (θ) of thedetector of Light-emitting Element 5-1.

FIG. 38 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 6.

FIG. 39 is a graph showing the measured and calculated integratedintensity of the EL emission spectrum depending on the angle (θ) of thedetector of Light-emitting Element 6-1.

FIG. 40 is a 2D contour map showing the measured angle dependence of theEL emission spectrum of Light-emitting Element 6-1.

FIG. 41 is a 2D contour map obtained by calculation of Light-emittingElement 6-1.

FIG. 42 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 7.

FIG. 43 is a graph showing the measured and calculated integratedintensity of the EL emission spectrum depending on the angle (θ) of thedetector of Light-emitting Element 7-1.

FIG. 44 is a 2D contour map showing the measured angle dependence of theEL emission spectrum of Light-emitting Element 7-1.

FIG. 45 is a 2D contour map obtained by calculation of Light-emittingElement 7-1.

FIG. 46 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 8.

FIG. 47 is a graph showing the measured and calculated integratedintensity of the EL emission spectrum depending on the angle (θ) of thedetector of Light-emitting Element 8-1.

FIG. 48 is a 2D contour map showing the measured angle dependence of theEL emission spectrum of Light-emitting Element 8-1.

FIG. 49 is a 2D contour map obtained by calculation of Light-emittingElement 8-1.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the drawings. Note that one embodiment of the presentinvention is not limited to the following description, and it will bereadily appreciated by those skilled in the art that the modes anddetails can be changed in various ways without departing from the spiritand the scope of the present invention. Thus, the present inventionshould not be construed as being limited to the description in thefollowing embodiments.

The process from carrier recombination to light emission of an organicEL element that emits fluorescence is described with reference toFIG. 1. First, voltage is applied to the organic EL element, and holesand electrons are injected from an anode and a cathode, respectively,into an EL layer. The injected carriers (holes and electrons) aretransported in the EL layer to a light-emitting layer and recombined ina certain area. The proportion of this recombination is called carrierbalance (γ). An organic material is excited by energy due to the carrierrecombination, and the generation ratio of singlet excitons to tripletexcitons is 1:3. This ratio is referred to as the proportion ofgenerated singlet excitons (α). Singlet excitons generated in alight-emitting material emit light in the fluorescent quantum yield(ϕ_(f)) of the organic compound. Singlet excitons generated in otherorganic compounds give energy to the light-emitting material, and thenthe singlet excitons in the organic compound emit light in thefluorescent quantum yield (ϕ_(f)). The proportion of light that isemitted in this manner and observed outside the light-emitting elementis the light extraction efficiency (χ) of the organic EL element. Theexternal quantum efficiency (μ_(ext)) of a fluorescent light-emittingelement is a product of the carrier balance (γ), the proportion ofgenerated singlet excitons (α), the fluorescent quantum yield (ϕ_(f)),and the light extraction efficiency (χ) and represented by Formula (1).[Formula 1]External quantum efficiency(μ_(ext))=χ·ϕ_(f)·α·γ  (1)

In the above formula, ϕ_(f) is a value depending on a light-emittingmaterial, and thus, light-emitting materials have their respectivevalues for ϕ_(f). In addition, γ can be assumed to be substantially 1 inan EL element having a stacked structure. Therefore, when the samematerial is used, terms which can be adjusted to improve the emissionefficiency depending on the element structure correspond to thefollowing two terms, i.e., the light extraction efficiency (χ) and theproportion of generated singlet excitons (α).

The proportion of generated singlet excitons (α) can be improved by amechanism which can up-convert a triplet exciton to a singlet exciton,such as triplet-triplet annihilation (TTA).

The light extraction efficiency (χ) is generally 20% to 30% in anorganic EL element over a glass substrate, although it depends on thestructure or stacked layers of a light-emitting device. However, theabove value is based on the assumption that light emission is isotropic;therefore, this value changes when light emission is anisotropic.

Light emission of a light-emitting material is generated in thedirection perpendicular to the transition dipole moment of a molecule.Accordingly, the light extraction efficiency (χ) can be improved bycontrolling the orientation state of the molecule.

As a method for evaluating the orientation state of a molecule in anamorphous organic thin film, a method using spectroscopic ellipsometryis given. In this method, the refractive index (n) and the extinctioncoefficient (k) of an organic material are measured to roughly analyzethe orientation state. It is practically reported that a long linearmolecule or a planar molecule in a material used for a light-emittingelement has an orientation parallel to the surface of a thin film (seeNon-Patent Document 1).

However, a practical light-emitting element has a stacked structure of aplurality of organic thin films, and a small amount of a light-emittingmaterial is dispersed in a host material. Therefore, the extinctioncoefficient (k) of a light-emitting material cannot be obtainedaccurately in a practical light-emitting element, and it is difficult toevaluate the orientation state of a molecule by the above method in thecase where the concentration of a light-emitting material is lower thanor equal to 10 wt % in a light-emitting layer.

In view of the above, the inventors of the present invention employed amethod for estimating the orientation state of a molecule according tothe emission state of a light-emitting element. The radiation angledependence of the emission intensity (spatial emission pattern) of thelight-emitting element depends on the direction of an average transitiondipole of the light-emitting material. If this spatial distribution canbe analyzed, the orientation state of the light-emitting element can beobtained. In this method, light emission of the light-emitting elementis observed and analyzed; thus, as long as the light-emitting materialemits light, it is possible to obtain the orientation state of thelight-emitting material in the light-emitting layer even when theconcentration of the material is low.

In practice, the measured angle dependence of the emission intensity andthe angle dependence of the emission intensity calculated by assuming,with a device simulator, a parameter a (see Formula (2) below) whichrepresents the orientation state of a light-emitting molecule arecompared; in this manner, an appropriate value of the parameter a whichrepresents the orientation state of a molecule can be estimated toobtain the orientation state of a light-emitting substance in alight-emitting element (see Non-Patent Document 2). The inventors of thepresent invention also focused their attention to the shape of theemission spectrum obtained by the device simulator, and compared themeasured and calculated values of the shape of the emission spectrum anda change in the shape of the emission spectrum depending on the angle topredict appropriate values. As the emission intensity in the measurementand simulation, not the emission intensity at a particular wavelengthbut the integrated intensity of the emission spectrum is used. By thesemethods which the inventors of the present invention employed, theparameter a can be estimated highly accurately, unlike in the methoddisclosed in Non-Patent Document 2.

Next, the parameter a which represents the orientation state of amolecule is described. FIG. 2 shows a relation between the observationdirection of a measurement device in measuring the spatial distributionof the emission intensity and components of transition dipoles which areorthogonal to each other over a substrate. As shown in FIG. 2, lightemission from average transition dipoles in a light-emitting material ina light-emitting layer (light emission practically observed from alight-emitting element) is decomposed into light emission fromtransition dipoles of a component in the x-axis direction (TEhcomponent), a component in the y-axis direction (TMh component), and acomponent in the z-axis direction (TMv component). Note that thesedirections are orthogonal to each other. In other words, it is assumedthat three kinds of orientation states of transition dipoles in thex-axis direction, the y-axis direction, and the z-axis direction existat a certain ratio. In that case, an emission pattern of alight-emitting element is determined by adding light emission from thesetransition dipoles. As described later, the parameter a relates to theabove ratio.

Light is emitted from the molecule in the direction perpendicular to thetransition dipole moment (a direction in a perpendicular plane) asdescribed above. Among the components divided into three directions, theTEh component and the TMh component (the x-axis direction and the y-axisdirection) are transition dipoles parallel to the substrate surface andtheir emission directions are perpendicular to the substrate, so thatlight emission from the TEh component and the TMh component can beeasily extracted. On the other hand, the TMv component (the z-axisdirection) is a transition dipole perpendicular to the substrate surfaceand its emission direction is parallel to the substrate, so that lightemission from the TMv component is not easily extracted.

In FIG. 2, a figure which extends from the center of an arrow thatrepresents the orientation of the transition dipole of each component isa schematic figure which represents the emission intensity that isdetected by the detector, when the direction of the detector is changedfrom front of the substrate (θ=0°) to parallel to the substrate (θ=90°).The vertical distance from the center is proportional to the intensity.

Since the detector is located in the direction in which light isemitted, the intensity of detected light of the TEh component (i.e., thevertical distance from the center of the arrow of the figure whichextends from the center of the arrow in FIG. 2) is constant, even whenthe angle of the substrate is changed, and the figure which extends fromthe center of the arrow has a fan shape. On the other hand, the figureswhich extend from the centers of the arrows of the TMh component and theTMv component have distorted fan shapes, which indicates that theintensity of detected light is greatly changed depending on the angle θof the detector to the substrate. As shown in FIG. 2, the TMh componenthas high intensity when θ is small (in a direction closer to the frontdirection of the substrate), whereas the TMv component has highintensity when θ is large (in a direction closer to the directionparallel to the substrate). In that case, the emission intensitymeasured by the measurement device (the emission intensity at awavelength λ and at an angle θ: I_(λ) (θ,λ)) can be represented byFormula (2).[Formula 2]I _(λ)(θ,λ)=a·I _(TMv)+(1−a)·(I _(TMh) +I _(TEh))  (2)

In the above formula, I_(TMv), I_(TMh), and I_(TEh) represent spatialintensity distribution of light emitted from the transition dipolesarranged as shown in FIG. 2, and a represents the proportion oftransition dipoles arranged perpendicular to the film surface (the TMvcomponent). In addition, 1−a represents the proportion of transitiondipoles arranged parallel to the film surface (the TMh and TEhcomponents). That is, a can also be regarded as a parameter whichrepresents the orientation of transition dipoles of light-emittingmolecules.

In the above formula, when the transition dipoles are arranged only inthe direction completely parallel to the substrate, the TMv component iseliminated and a is 0. In contrast, when the transition dipoles arearranged only in the direction perpendicular to the substrate, a is 1.When the directions of the transition dipoles are not the same, theratio of the components of the transition dipoles is supposed to beisotropic, i.e., x-axis:y-axis:z-axis=1:1:1, so that the ratio of thecomponent perpendicular to the substrate (the TMv component) to thecomponents parallel to the substrate (the TMh and TEh components) is 1:2and a is ⅓ (approximately 0.33).

As described above, I_(TEh) is constant independent of the angle;however, I_(TMh) and I_(TMv) change depending on the angle (θ) of thesubstrate to the measurement device. Thus, by measuring the emissionintensity while changing θ, a can be obtained from the change in I_(TMh)and I_(TMv) depending on θ.

In that case, I_(TEh) which does not change depending on the anglehinders the measurement. The amplitude direction of an electric field ofemitted light is the same as the direction of the transition dipolemoment, and I_(TEh) is an S-wave and I_(TMv) and I_(TMh) are P-waves.Thus, by disposing a linear polarizer in the direction perpendicular tothe substrate surface, measurement can be performed under the conditionwhere the TEh component is excluded.

The TMh component and the TMv component are compared. The emissiondirection of the TMh component is mainly perpendicular to the substrateand the emission direction of the TMv component is mainly parallel tothe substrate; in a light-emitting element in which light emission isobtained from a solid, a large part of light emission from the TMvcomponent is totally reflected and cannot be extracted to the outside.On the other hand, light emission from the TMh component is more easilyextracted to the outside than light emission from the TMv component.Furthermore, in a light-emitting element in which the thickness isoptically optimized, light emission from the TMh component whoseemission direction is mainly perpendicular to the substrate isintensified through interference, so that the emission intensity of theTMh component is increased (thus, the emission efficiency is maximized).That is, unless the parameter a representing the orientation is veryclose to 1, a difference between the emission intensity of the TMvcomponent and that of the TMh component is very large in thelight-emitting element in which the thickness is optically optimized.That is, in the light-emitting element in which the emission efficiencyis maximized, most light emission which is observed depends on the TMhcomponent. In the case where the difference between the emissionintensity of the TMh component and that of the TMv component is large asdescribed above, it is difficult to experimentally extract lightemission from the component which has lower intensity (namely the TMvcomponent) from the distribution of the emission intensity depending onthe angle.

Thus, in this embodiment, the emission intensity in the front directionof the substrate is suppressed as much as possible by utilizing aninterference effect (that is, light emission from the TMh component isreduced as much as possible by utilizing an interference effect), sothat the ratio of the TMh component to the TMv component, i.e., theparameter a, can be easily obtained. For this purpose, an element inwhich the thickness is adjusted is prepared for measurement.Specifically, an element is fabricated and used for measurement, inwhich the luminance in the front direction of the substrate is loweredby setting the distance between a light-emitting region and a cathode tonλ/2. In general, the thickness is adjusted in such a manner that thethickness of an electron-transport layer to which an alkali metal isadded is increased. However, since there is a limitation on theconductivity of the film, the drive voltage might be increased or thecarrier balance might be poor. Accordingly, in order to adjust thethickness, it is preferable to use a composite material of a materialhaving a hole-transport property and a material having an acceptorproperty with respect to the material having a hole-transport property.It is preferable to use the composite material for a hole-injectionlayer in an EL layer or a layer between an electron-injection layer anda cathode.

As the material having a hole-transport property used for the compositematerial, any of a variety of organic compounds such as aromatic aminecompounds, carbazole derivatives, aromatic hydrocarbons, and highmolecular compounds (e.g., oligomers, dendrimers, or polymers) can beused. Note that the substance having a hole-transport property which isused for the composite material is preferably a substance having a holemobility of 10⁻⁶ cm²/Vs or higher. Examples of organic compounds thatcan be used as the material having a hole-transport property in thecomposite material are specifically given below.

Examples of the material having a hole-transport property that can beused for the composite material include aromatic amine compounds such asN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD), and1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B); carbazole derivatives such as3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), 4,4′-di(N-carbazolyl)biphenyl (abbreviation:CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), and1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene; and aromatichydrocarbons such as 2-tert-butyl-9,10-di(2-naphthyl)anthracene(abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, and 2,5,8,11-tetra(tert-butyl)perylene.Other examples are pentacene, coronene, and the like. Aromatichydrocarbon having a vinyl skeleton may also be used and examplesthereof are 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi),9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA),and the like.

In particular, as the material having a hole-transport property, adibenzothiophene derivative or a dibenzofuran derivative such as4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV), 1,3,5-tri(dibenzothiophen-4-yl)-benzene(abbreviation: DBT3P-II), 4,4′-(biphenyl-2,2′-diyl)-bis-dibenzothiophene(abbreviation: oDBTBP-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III),4-[3-(triphenylen-2-yl)phenyl]dibenzothiophene (abbreviation:mDBTPTp-II), 3,6-di-(dibenzothiophen-4-yl)-9-phenyl-9H-carbazole(abbreviation: DBT2PC-II),4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzothiophene (abbreviation:2mDBTPPA-II), 4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran(abbreviation: 2mDBFPPA-II), or4-[4-(9-phenylanthracen-10-yl)phenyl]dibenzothiophene (abbreviation:mDBTPA-II), or a hydrocarbon compound in which a substituent is bondedto a naphthalene skeleton, a phenanthrene skeleton, or a triphenyleneskeleton and in which the molecular weight is 350 to 2000, such as1-[3,5-di(naphthalen-1-yl)phenyl]naphthalene (abbreviation: N3P),9-[3,5-di(phenanthren-9-yl)phenyl]phenanthrene (abbreviation: Pn3P),1,2,3,4-tetraphenylnaphthalene (abbreviation: P4N),2-[3,5-di-(naphthalen-2-yl)-phenyl]-naphthalene (abbreviation: (3N3P),or 9,9′-(biphenyl-3,3′-diyl)-di-phenanthrene (abbreviation: mPnBP) canbe used. A composite material including such a material exhibits noabsorption ranging from a visible light region to a near-infraredregion. The measurement results of a light-emitting element containingthe composite material almost correspond to the calculation results,that is, a can be accurately obtained.

Other examples are high molecular compounds such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N-[4-(4-diphenylamino)phenyl]phenyl-N-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:poly-TPD).

As examples of the substance having an acceptor property,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, atransition metal oxide can be given. In addition, an oxide of metalsthat belong to Group 4 to Group 8 of the periodic table can be given.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable since their electron-accepting property is high.Among these, molybdenum oxide is especially preferable since it isstable in the air and low in hygroscopic property and is thus easilytreated.

The composite material has high conductivity and thus has an advantagein that the drive voltage is unlikely to be increased and the carrierbalance can be kept even when the thickness of a film containing thecomposite material is large.

As described above, a light-emitting element with a partly specialstructure is used for the measurement. However, by forming alight-emitting layer having a structure similar to that of a generallight-emitting element, the evaluation results of the orientation statecan also be applied to the orientation state of a light-emitting elementwith an ordinary structure.

Such a light-emitting element is made to emit light and a linearpolarizer is disposed in the direction perpendicular to the substrate;in this manner, the angle dependence of the emission intensity ismeasured. The emission intensity may be represented as an intensity at acertain wavelength but is preferably represented as a value obtained byintegrating the emission spectrum intensity in one embodiment of thepresent invention, because a more accurate examination can be performed.

This experimental value is compared with a calculation result obtainedby an organic device simulator (a semiconducting emissive thin filmoptics simulator, namely setfos, produced by Cybernet Systems Co.,Ltd.), so that a of the light-emitting element can be obtained. In thiscalculation, the spectrum shape of a light-emitting material, thethickness of a stacked structure, the refractive index, the extinctionefficiency, and the position and the width of a light-emitting regionare input to calculate the emission intensity (spectrum) depending onthe angle θ, which corresponds to an input given parameter a.

Note that the position of the light-emitting region cannot be measuredand is thus assumed. The position of the light-emitting region can beassumed empirically in consideration of a carrier-transport property orthe like of the light-emitting layer. However, the following method ispreferable to a method of fixing the position of the light-emittingregion to one position in the thickness direction: first, alight-emitting position where the recombination probability is supposedto be the highest (e.g., the vicinity of the interface between ahole-transport layer and the light-emitting layer when, for example, anelectron-transport property of the light-emitting layer is higher than ahole-transport property thereof) is fixed, and calculation is performedassuming that the light-emitting region spreads such that therecombination probability is decreased exponentially from thelight-emitting position. Through this method, favorable calculationresults of the spectrum shape close to the measured one can be obtained.

The inventors of the present invention found that a favorablelight-emitting element with extremely high emission efficiency can beobtained by setting a calculated by the above method to less than orequal to 0.2, preferably greater than or equal to 0 and less than orequal to 0.2.

The light extraction efficiency in each orientation state is discussed.As compared with the case where the transition dipole moment has arandom orientation (a=⅓≈0.33), transition dipoles of ⅓ of moleculeswhich have been perpendicular to the substrate in a random orientationare parallel to the substrate in the case where the transition dipolemoment has an orientation completely parallel to the substrate (a=0).Therefore, the proportion of the transition dipoles parallel to thesubstrate surface is 1.5 times the proportion in the random orientation.

As described above, most light emission observed in the optimizedlight-emitting element is derived from emission components of moleculesin the horizontal orientation, and light emission from molecules in theperpendicular orientation (i.e., the TMv component) is relatively weakso as to be negligible. In other words, it is suggested that, in thecase of the random orientation, light emission from ⅓ of molecules isnot extracted substantially. On the other hand, in the case of a=0, theproportion of transition dipoles parallel to the substrate is 1.5 timesthe proportion in the random orientation as described above, so that theproportion of molecules contributing to observed light emission and thelight extraction efficiency are also approximately 1.5 times those inthe random orientation.

As described above, in the light-emitting element of one embodiment ofthe present invention, by setting a to less than or equal to 0.2, morelight emission can be extracted to the outside than in the randomorientation, so that a light-emitting element with high external quantumefficiency can be provided. In the case of a=0.2, the proportion oftransition dipoles in the horizontal orientation is 1.2 times theproportion in the random orientation, so that the light extractionefficiency can also be 1.2 times the one in the random orientation.

Note that an electrode of the light-emitting element is providedparallel to the substrate, so that a transition dipole parallel to thesubstrate is parallel to a first electrode or a second electrode of thelight-emitting element.

A fluorescent light-emitting element in which a light-emitting materialis a fluorescent substance has been described as an example; however,the above description can also apply to a phosphorescent light-emittingelement in which a light-emitting material is a phosphorescentsubstance. When a light-emitting material is a phosphorescent substance,a light-emitting element with extremely high emission efficiency can beobtained. A phosphorescent substance is preferably an iridium complex.In a phosphorescent light-emitting element including a light-emittinglayer in which a is less than or equal to 0.2, the efficiency can beeasily high, that is, the external quantum efficiency can be easilyhigher than or equal to 25%. When the phosphorescent quantum yield ofthe light-emitting material is very high (e.g., higher than or equal to0.84, preferably higher than or equal to 0.9), it is possible to providea light-emitting element which has extremely high efficiency, that is,an external quantum efficiency that exceeds the theoretical limit,higher than or equal to 30%. In order to efficiently transfer energy orto reduce the drive voltage, the following structure of a phosphorescentlight-emitting element is preferable: a light-emitting layer contains athird substance in addition to a host material and a light-emittingmaterial, and the host material and the third substance form anexciplex.

When a light-emitting material is a fluorescent material, the materialis preferably a substance having a condensed aromatic hydrocarbonskeleton for the molecular orientation. A light-emitting elementincluding a light-emitting layer in which a is less than or equal to 0.2has a light extraction efficiency which is increased 1.2 times. When thefluorescent quantum yield of a light-emitting material of a fluorescentlight-emitting element is very high (e.g., higher than or equal to 0.84,preferably higher than or equal to 0.9), it is possible to provide alight-emitting element which has high efficiency, that is, an externalquantum efficiency that exceeds the theoretical limit, higher than orequal to 7.5%. A light-emitting element having a mechanism such as TTAin addition to the above structure can have very high efficiency, thatis, an external quantum efficiency of 10% or higher. In such alight-emitting element also having TTA, a delayed fluorescent componentis observed.

<<Light-Emitting Element>>

Next, an example of a light-emitting element of one embodiment of thepresent invention is described in detail below with reference to FIG.3A.

In this embodiment, the light-emitting element includes a pair ofelectrodes (a first electrode 101 and a second electrode 102), and an ELlayer 103 provided between the first electrode 101 and the secondelectrode 102. Note that the first electrode 101 functions as an anodeand the second electrode 102 functions as a cathode.

To function as an anode, the first electrode 101 is preferably formedusing any of metals, alloys, conductive compounds having a high workfunction (specifically, a work function of 4.0 eV or higher), mixturesthereof, and the like. Specific examples include indium oxide-tin oxide(ITO: indium tin oxide), indium oxide-tin oxide containing silicon orsilicon oxide, indium oxide-zinc oxide, and indium oxide containingtungsten oxide and zinc oxide (IWZO). Films of these electricallyconductive metal oxides are usually formed by a sputtering method butmay also be formed by application of a sol-gel method or the like. Forexample, a film of indium oxide-zinc oxide is formed by a sputteringmethod using a target obtained by adding 1 wt % to 20 wt % of zinc oxideto indium oxide. A film of indium oxide containing tungsten oxide andzinc oxide (IWZO) can be formed by a sputtering method using a target inwhich tungsten oxide and zinc oxide are added to indium oxide at 0.5 wt% to 5 wt % and 0.1 wt % to 1 wt %, respectively. Other examples aregold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), anitride of a metal material (such as titanium nitride), and the like.Graphene can also be used. Note that when a composite material describedlater is used for a layer which is in contact with the first electrode101 in the EL layer 103, an electrode material can be selectedregardless of its work function.

The EL layer 103 has a stacked structure and includes at least alight-emitting layer. Examples of layers included in the EL layer 103other than the light-emitting layer are a hole-injection layer, ahole-transport layer, an electron-transport layer, an electron-injectionlayer, a carrier-blocking layer, and an intermediate layer. Thelight-emitting element can be formed by combining these layers asappropriate. In this embodiment, the EL layer 103 has a structure inwhich a hole-injection layer 111, a hole-transport layer 112, alight-emitting layer 113, an electron-transport layer 114, and anelectron-injection layer 115 are stacked in this order over the firstelectrode 101. Specific examples of the materials forming the layers aregiven below.

The hole-injection layer 111 is a layer containing a substance having ahigh hole-injection property. The hole-injection layer 111 can be formedusing molybdenum oxide, vanadium oxide, ruthenium oxide, tungsten oxide,manganese oxide, or the like. Alternatively, the hole-injection layer111 can be formed using a phthalocyanine-based compound such asphthalocyanine (abbreviation: H₂Pc) or copper phthalocyanine(abbreviation: CuPc), an aromatic amine compound such as4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB) orN,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD), a high molecule such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, or the like.

Alternatively, a composite material in which a substance having ahole-transport property contains a substance having an acceptor propertycan be used for the hole-injection layer 111. This composite material isthe same as the above-described composite material which is preferablyused for adjusting the thickness of the light-emitting element; thus,its description is omitted. By using the composite material for thehole-injection layer, the material of the first electrode can beselected regardless of its work function.

By providing the hole-injection layer 111, a high hole-injectionproperty can be achieved to allow the light-emitting element to bedriven at low voltage.

The hole-transport layer 112 is a layer containing a substance having ahole-transport property. Examples of the substance having ahole-transport property include aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N′-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), and4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP).The substances listed here have high hole-transport properties and aremainly ones that have a hole mobility of 10⁻⁶ cm²/Vs or higher. Anorganic compound given as an example of the substance having ahole-transport property in the composite material described above canalso be used for the hole-transport layer 112. Note that the layer thatcontains a substance having a hole-transport property is not limited toa single layer, and may be a stack of two or more layers including anyof the above substances.

The light-emitting layer 113 may be a layer that emits fluorescence, alayer that emits phosphorescence, or a layer that emits thermallyactivated delayed fluorescence (TADF). Furthermore, the light-emittinglayer 113 may be a single layer or include a plurality of layerscontaining different light-emitting substances. In the case where thelight-emitting layer including a plurality of layers is formed, a layercontaining a phosphorescent substance and a layer containing afluorescent substance may be stacked. In that case, an exciplexdescribed later is preferably utilized in the layer containing aphosphorescent substance.

As a fluorescent substance, any of the following substances can be used,for example. Fluorescent substances other than those given below canalso be used. Specific examples includeN,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn),N,N′-diphenyl-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6mFLPAPrn),N,N′-bis(2,6-dimethylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6oDMemFLPAPrn),N,N′-bis[4-(dibenzofuran-4-yl)phenyl]-N,N′-diphenyl-pyrene-1,6-diamine(abbreviation: 1,6FrBAPrn-II),N,N′-bis[3-(dibenzofuran-4-yl)phenyl]-N,N′-diphenyl-pyrene-1,6-diamine(abbreviation: 1,6mFrBAPrn-II),N,N′-(pyrene-1,6-diyl)bis[(N-phenylbenzo[b]naphtho[1,2-d]furan)-8-amine](abbreviation:1,6BnfAPrn-02),N,N′-(pyrene-1,6-diyl)bis[(6,N-diphenylbenzo[b]naphtho[1,2-d]furan)-8-amine](abbreviation:1,6BnfAPrn-03),9,10-bis[N-phenyl-N-(9-phenylcarbazol-3-yl)amino]anthracene(abbreviation: PCA2A), and coumarin 545T. Condensed aromatic diaminecompounds typified by pyrenediamine compounds, such as 1,6FLPAPrn and1,6mMemFLPAPrn, are preferable because of their high hole-trappingproperties, high emission efficiency, high reliability, and easiness inthe molecular orientation.

Examples of a material which can be used as a phosphorescent substancein the light-emitting layer 113 are as follows: an organometalliciridium complex having an azole (in particular, a triazole or imidazole)skeleton, such astris{2-[4-(2-adamantyl)-3-methyl-4H-1,2,4-triazol-5-yl-κN]phenyl-κC}iridium(III)(abbreviation: [Ir(Mptz-Adm2)₃]),tris{2-[4-(4-cyano-2,6-diisobutylphenyl)-5-(2-methylphenyl)-4H-1,2,4-triazol-3-yl-κN²]phenyl-κC}iridium(III)(abbreviation: Ir(mpptz-diBuCNp)₃),tris{2-[4-(4-cyano-2,6-dimethylphenyl)-5-(2-methylphenyl)-4H-1,2,4-triazol-3-yl-κN²]phenyl-κC}iridium(III)(abbreviation: Ir(mpptz-dmCNp)₃), ortris{2-[1-(4-cyano-2,6-diisobutylphenyl)-1H-imidazol-2-yl-κN³]phenyl-κC}iridium(III)(abbreviation: Ir(pim-diBuCNp)₃), an organometallic iridium complexhaving a pyrimidine skeleton, such as(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]),(acetylacetonato)bis[6-(2-norbornyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: [Ir(nbppm)₂(acac)]),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: [Ir(mpmppm)₂(acac)]),(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]),(acetylacetonato)bis(4,5,6-triphenylpyrimidinato)iridium(III)(abbreviation: [Ir(tppm)₂(acac)]),bis{2-[6-(3,5-dimethylphenyl)-4-pyrimidinyl-κN3]-4,6-dimethylphenyl-κC}(2,6-dimethyl-3,5-heptanedionato-κ²O,O′)iridium(III) (abbreviation:[Ir(dmdppm)₂(dibm)]), orbis{2-[6-(3,5-dimethylphenyl)-4-pyrimidinyl-κN3]-4,6-dimethylphenyl-κC}(2,2′,6,6′-tetramethyl-3,5-heptanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppm)₂(dpm)]), and an organometallic iridiumcomplex having a pyrazine skeleton, such asbis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(tppr)₂(dpm)]),bis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-dmp)₂(acac)]),bis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,8-dimethyl-4,6-nonanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-dmp)₂(divm)]),bis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,2′,6,6′-tetramethyl-3,5-heptanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-dmp)₂(dpm)]), orbis{4,6-dimethyl-2-[5-(2,5-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,2,6,6-tetramethyl-3,5-heptanedionato-k²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-25dmp)₂(dpm)]). These organometallic iridiumcomplexes are preferable because of their high emission efficiency, highreliability, and easiness in the molecular orientation.

As well as the above phosphorescent compounds, a variety ofphosphorescent materials may be selected and used.

Examples of the TADF material include a heterocyclic compound havingboth a π-electron rich heteroaromatic ring and a t-electron deficientheteroaromatic ring, such as2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine(abbreviation: PIC-TRZ),9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9′-phenyl-9H,9′H-3,3′-bicarbazole(abbreviation: PCCzTzn),2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine(abbreviation: PCCzPTzn),2-[4-(10H-phenoxazine-10-yl)phenyl]-4,6-diphenyl-1,3,5-triazine(abbreviation: PXZ-TRZ),3-[4-(5-phenyl-5,10-dihydrophenazin-10-yl)phenyl]-4,5-diphenyl-1,2,4-triazole(abbreviation: PPZ-3TPT),3-(9,9-dimethyl-9H-acridin-10-yl)-9H-xanthen-9-one (abbreviation:ACRXTN), bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone(abbreviation: DMAC-DPS), or10-phenyl-10H,10′H-spiro[acridin-9,9′-anthracen]-10′-one (abbreviation:ACRSA). The heterocyclic compound is preferable because of having highelectron-transport and hole-transport properties owing to a π-electronrich heteroaromatic ring and a π-electron deficient heteroaromatic ring.Note that a substance in which the π-electron rich heteroaromatic ringis directly bonded to the π-electron deficient heteroaromatic ring isparticularly preferably used because the donor property of thet-electron rich heteroaromatic ring and the acceptor property of theπ-electron deficient heteroaromatic ring are both increased, the energydifference between the S₁ level and the T₁ level becomes small, and thusthermally activated delayed fluorescence can be obtained with highefficiency. Note that an aromatic ring to which an electron-withdrawinggroup such as a cyano group is bonded may be used instead of theπ-electron deficient heteroaromatic ring.

As a host material of the light-emitting layer, variouscarrier-transport materials such as a material having anelectron-transport property or a material having a hole-transportproperty can be used.

Examples of the material having an electron-transport property include ametal complex such as bis(10-hydroxybenzo[h]quinolinato)beryllium(II)(abbreviation: BeBq₂), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq),bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); aheterocyclic compound having a polyazole skeleton, such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), or2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); a heterocyclic compound having a diazineskeleton, such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[fh]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[fh]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine(abbreviation: 4,6mPnP2Pm), or4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); and a heterocyclic compound having a pyridine skeleton,such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:35DCzPPy) or 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation:TmPyPB). Among the above materials, a heterocyclic compound having adiazine skeleton and a heterocyclic compound having a pyridine skeletonhave high reliability and are thus preferable. Specifically, aheterocyclic compound having a diazine (pyrimidine or pyrazine) skeletonhas a high electron-transport property to contribute to a reduction indrive voltage.

Examples of materials having a hole-transport property include acompound having an aromatic amine skeleton, such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF), orN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); a compound having a carbazole skeleton, such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound havinga thiophene skeleton, such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), or4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and a compound having a furan skeleton, suchas 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation:DBF3P-II) or4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, a compoundhaving an aromatic amine skeleton and a compound having a carbazoleskeleton are preferable because these compounds are highly reliable andhave high hole-transport properties to contribute to a reduction indrive voltage. Hole-transport materials can be selected from a varietyof substances as well as from the hole-transport materials given above.

In the case of using a fluorescent substance as a light-emittingsubstance, materials having an anthracene skeleton, such as9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA), 3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation:PCPN), 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation:CzPA), 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA),6-[3-(9,10-diphenyl-2-anthryl)phenyl]-benzo[b]naphtho[1,2-d]furan(abbreviation: 2mBnfPPA), and9-phenyl-10-{4-(9-phenyl-9H-fluoren-9-yl)-biphenyl-4′-yl}-anthracene(abbreviation: FLPPA), are preferably used. The use of a substancehaving an anthracene skeleton as a host material for a fluorescentsubstance makes it possible to obtain a light-emitting layer with highemission efficiency and high durability. In particular, CzPA, cgDBCzPA,2mBnfPPA, and PCzPA are preferable because of their excellentcharacteristics.

Note that a host material may be a mixture of a plurality of kinds ofsubstances, and in the case of using a mixed host material, it ispreferable to mix a material having an electron-transport property witha material having a hole-transport property. By mixing the materialhaving an electron-transport property with the material having ahole-transport property, the transport property of the light-emittinglayer 113 can be easily adjusted and a recombination region can beeasily controlled. The ratio of the content of the material having ahole-transport property to the content of the material having anelectron-transport property may be 1:9 to 9:1.

These mixed host materials may form an exciplex. When a combination ofthese materials is selected so as to form an exciplex that exhibitslight emission whose wavelength overlaps with the wavelength of alowest-energy-side absorption band of the fluorescent substance, thephosphorescent substance, or the TADF material, energy is transferredsmoothly and light emission can be obtained efficiently. Such astructure is preferable in that the drive voltage can be reduced.

Since a which represents the orientation state of a light-emittingmaterial is unlikely to be influenced by the kind or number of hostmaterials, any material may be selected as a host material.

The light-emitting layer 113 having the above-described structure can beformed by co-evaporation using a vacuum evaporation method. In thatcase, an effective atmosphere for a chamber is as follows: the ratio ofthe partial pressure of carbon dioxide with respect to the totalpressure in the chamber, which is measured by a quadrupole mass analyzer(Q-mass) disposed in the evaporation chamber, is higher than that in theair. In the air, the percentage of the partial pressure of carbondioxide with respect to the total pressure (i.e., volume ratio) isapproximately 0.03%. A light-emitting element having an orientationstate in which a of a light-emitting material is less than or equal to0.2 can be fabricated in such a manner that a light-emitting layer isformed in a vacuum chamber in a reduced pressure atmosphere in the statewhere the percentage of the partial pressure of carbon dioxide withrespect to the total pressure is higher than 0.03%, preferably higherthan or equal to 0.1%. Since carbon dioxide hinders a carrier-transportproperty, its percentage is preferably lower than or equal to 10%.

The electron-transport layer 114 contains a substance having anelectron-transport property. As the substance having anelectron-transport property, the materials having an electron-transportproperty or having an anthracene skeleton, which are described above asmaterials for the host material, can be used.

A layer for controlling the transport of electron carriers may beprovided between the electron-transport layer and the light-emittinglayer. This is a layer formed by addition of a small amount of asubstance having a high electron-trapping property to a material havinga high electron-transport property as described above, and the layer iscapable of adjusting the carrier balance by suppressing the transport ofelectron carriers. Such a structure is very effective in preventing aproblem (such as a reduction in element lifetime) caused when electronspass through the light-emitting layer.

In addition, the electron-injection layer 115 may be provided in contactwith the second electrode 102, between the electron-transport layer 114and the second electrode 102. For the electron-injection layer 115, analkali metal, an alkaline earth metal, or a compound thereof, such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂), can be used. For example, a layer that is formed using asubstance having an electron-transport property and contains an alkalimetal, an alkaline earth metal, or a compound thereof can be used. Anelectride may also be used for the electron-injection layer 115.Examples of the electride include a substance in which electrons areadded at high concentration to calcium oxide-aluminum oxide. Note that alayer that is formed using a substance having an electron-transportproperty and contains an alkali metal or an alkaline earth metal ispreferably used for the electron-injection layer 115, in which caseelectrons are efficiently injected from the second electrode 102.

Instead of the electron-injection layer 115, a charge-generation layer116 may be provided (FIG. 3B). The charge-generation layer 116 refers toa layer capable of injecting holes into a layer in contact with thecathode side of the charge-generation layer 116 and electrons into alayer in contact with the anode side thereof when a potential isapplied. The charge-generation layer 116 includes at least a p-typelayer 117. The p-type layer 117 is preferably formed using any of thecomposite materials given above as examples of materials that can beused for the hole-injection layer 111. The p-type layer 117 may beformed by stacking a film containing the above-described acceptormaterial as a material included in the composite material and a filmcontaining a hole-transport material. When a potential is applied to thep-type layer 117, electrons are injected into the electron-transportlayer 114 and holes are injected into the second electrode 102functioning as a cathode; thus, the light-emitting element operates.

Note that the charge-generation layer 116 preferably includes either anelectron-relay layer 118 or an electron-injection buffer layer 119 orboth in addition to the p-type layer 117.

The electron-relay layer 118 contains at least the substance having anelectron-transport property and has a function of preventing aninteraction between the electron-injection buffer layer 119 and thep-type layer 117 and smoothly transferring electrons. The LUMO level ofthe substance having an electron-transport property contained in theelectron-relay layer 118 is preferably between the LUMO level of thesubstance having an acceptor property in the p-type layer 117 and theLUMO level of a substance contained in a layer of the electron-transportlayer 114 in contact with the charge-generation layer 116. As a specificvalue of the energy level, the LUMO level of the substance having anelectron-transport property in the electron-relay layer 118 ispreferably higher than or equal to −5.0 eV, more preferably higher thanor equal to −5.0 eV and lower than or equal to −3.0 eV. Note that as thesubstance having an electron-transport property in the electron-relaylayer 118, a phthalocyanine-based material or a metal complex having ametal-oxygen bond and an aromatic ligand is preferably used.

A substance having a high electron-injection property can be used forthe electron-injection buffer layer 119. For example, an alkali metal,an alkaline earth metal, a rare earth metal, or a compound thereof(e.g., an alkali metal compound (including an oxide such as lithiumoxide, a halide, and a carbonate such as lithium carbonate or cesiumcarbonate), an alkaline earth metal compound (including an oxide, ahalide, and a carbonate), or a rare earth metal compound (including anoxide, a halide, and a carbonate)) can be used.

In the case where the electron-injection buffer layer 119 contains thesubstance having an electron-transport property and a donor substance,an organic compound such as tetrathianaphthacene (abbreviation: TTN),nickelocene, or decamethylnickelocene can be used as the donorsubstance, as well as an alkali metal, an alkaline earth metal, a rareearth metal, or a compound of the above metal (e.g., an alkali metalcompound (including an oxide such as lithium oxide, a halide, and acarbonate such as lithium carbonate or cesium carbonate), an alkalineearth metal compound (including an oxide, a halide, and a carbonate),and a rare earth metal compound (including an oxide, a halide, and acarbonate)).

For the second electrode 102, any of metals, alloys, electricallyconductive compounds having a low work function (specifically, a workfunction of 3.8 eV or lower), and mixtures thereof, and the like can beused. Specific examples of such a cathode material include elementsbelonging to Groups 1 and 2 of the periodic table, such as alkali metals(e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), andstrontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metalssuch as europium (Eu) and ytterbium (Yb), and alloys thereof. However,when the electron-injection layer is provided between the secondelectrode 102 and the electron-transport layer, for the second electrode102, any of a variety of conductive materials such as Al, Ag, ITO, orindium oxide-tin oxide containing silicon or silicon oxide can be usedregardless of its work function. Films of these conductive materials canbe formed by a dry method such as a vacuum evaporation method or asputtering method, an inkjet method, a spin coating method, or the like.In addition, the films of these conductive materials may be formed by awet method using a sol-gel method, or by a wet method using paste of ametal material.

Any of various methods can be employed for forming the layers other thanthe light-emitting layer which are included in the EL layer 103regardless of whether it is a dry method or a wet method. For example, avacuum evaporation method, a gravure printing method, an offset printingmethod, a screen printing method, an inkjet method, a spin coatingmethod, or the like may be used.

Here, a method for forming an EL layer 786 by a droplet discharge methodis described with reference to FIGS. 4A to 4D. FIGS. 4A to 4D arecross-sectional views illustrating the method for forming the EL layer786.

First, a conductive film 772 is formed over a planarization insulatingfilm 770, and an insulating film 730 is formed to cover part of theconductive film 772 (see FIG. 4A).

Then, a droplet 784 is discharged to an exposed portion of theconductive film 772, which is an opening of the insulating film 730,from a droplet discharge apparatus 783, so that a layer 785 containing acomposition is formed. The droplet 784 is a composition containing asolvent and is attached to the conductive film 772 (see FIG. 4B).

Note that the step of discharging the droplet 784 may be performed underreduced pressure.

Next, the solvent is removed from the layer 785 containing acomposition, and the resulting layer is solidified to form the EL layer786 (see FIG. 4C).

The solvent may be removed by drying or heating.

Next, a conductive film 788 is formed over the EL layer 786; thus, alight-emitting element 782 is completed (see FIG. 4D).

When the EL layer 786 is formed by a droplet discharge method asdescribed above, the composition can be selectively discharged;accordingly, waste of material can be reduced. Furthermore, alithography process or the like for shaping is not needed, and thus, theprocess can be simplified and cost reduction can be achieved.

The droplet discharge method described above is a general term for ameans including a nozzle equipped with a composition discharge outlet ora means to discharge droplets, such as a head having one or a pluralityof nozzles.

Next, a droplet discharge apparatus used for the droplet dischargemethod is described with reference to FIG. 5. FIG. 5 is a conceptualdiagram illustrating a droplet discharge apparatus 1400.

The droplet discharge apparatus 1400 includes a droplet discharge means1403. The droplet discharge means 1403 is equipped with a head 1405, ahead 1412, and a head 1416.

The heads 1405 and 1412 are connected to a control means 1407, and thiscontrol means 1407 is controlled by a computer 1410; thus, apreprogrammed pattern can be drawn.

The drawing may be conducted at a timing, for example, based on a marker1411 formed over a substrate 1402. Alternatively, the reference pointmay be determined on the basis of an outer edge of the substrate 1402.Here, the marker 1411 is detected by an imaging means 1404 and convertedinto a digital signal by an image processing means 1409. Then, thedigital signal is recognized by the computer 1410, and then, a controlsignal is generated and transmitted to the control means 1407.

An image sensor or the like using a charge coupled device (CCD) or acomplementary metal oxide semiconductor (CMOS) can be used as theimaging means 1404. Note that information about a pattern to be formedover the substrate 1402 is stored in a storage medium 1408, and acontrol signal is transmitted to the control means 1407 on the basis ofthe information, so that each of the heads 1405, 1412, and 1416 of thedroplet discharge means 1403 can be individually controlled. A materialto be discharged is supplied to the heads 1405, 1412, and 1416 frommaterial supply sources 1413, 1414, and 1415, respectively, throughpipes.

Inside each of the heads 1405, 1412, and 1416, a space as indicated by adotted line 1406 to be filled with a liquid material and a nozzle whichis a discharge outlet are provided. Although not illustrated, an insidestructure of the head 1412 is similar to that of the head 1405. When thenozzle sizes of the heads 1405 and 1412 are different from each other,different materials with different widths can be dischargedsimultaneously. Each head can discharge and draw a plurality oflight-emitting materials. In the case of drawing over a large area, thesame material can be simultaneously discharged to be drawn from aplurality of nozzles in order to improve throughput. When a largesubstrate is used, the heads 1405, 1412, and 1416 can freely scan thesubstrate in the directions indicated by arrows X, Y, and Z in FIG. 5,and a region in which a pattern is drawn can be freely set. Thus, aplurality of the same patterns can be drawn over one substrate.

Furthermore, a step of discharging the composition may be performedunder reduced pressure. Also, a substrate may be heated when thecomposition is discharged. After discharging the composition, eitherdrying or baking or both is performed. Both the drying and baking areheat treatments but different in purpose, temperature, and time period.The steps of drying and baking are performed under normal pressure orunder reduced pressure by laser irradiation, rapid thermal annealing,heating using a heating furnace, or the like. Note that the timing ofthe heat treatment and the number of times of the heat treatment are notparticularly limited. The temperature for performing each of the stepsof drying and baking in a favorable manner depends on the material ofthe substrate and the properties of the composition.

In the above-described manner, the EL layer 786 can be formed by thedroplet discharge apparatus.

In the case where the EL layer 786 is formed by the droplet dischargeapparatus, the following various organic solvents can be used to form acoating composition: benzene, toluene, xylene, mesitylene,tetrahydrofuran, dioxane, ethanol, methanol, n-propanol, isopropanol,n-butanol, t-butanol, acetonitrile, dimethylsulfoxide,dimethylformamide, chloroform, methylene chloride, carbon tetrachloride,ethyl acetate, hexane, cyclohexane, and the like. In particular, lesspolar benzene derivatives such as benzene, toluene, xylene, andmesitylene are preferable because a solution with a suitableconcentration can be obtained and the material contained in ink can beprevented from deteriorating due to oxidation or the like. Furthermore,to achieve a uniform film or a film with a uniform thickness, a solventwith a boiling point of 100° C. or higher is preferably used, and morepreferably, toluene, xylene, or mesitylene is used.

Note that the above-described structure can be combined with any of thestructures in this embodiment.

In addition, the electrode may be formed by a wet method using a sol-gelmethod, or by a wet method using paste of a metal material. Furthermore,the electrode may be formed by a dry method such as a sputtering methodor a vacuum evaporation method.

Light emission from the light-emitting element is extracted out throughone or both of the first electrode 101 and the second electrode 102.Therefore, one or both of the first electrode 101 and the secondelectrode 102 are formed as a light-transmitting electrode.

The structure of the layers provided between the first electrode 101 andthe second electrode 102 is not limited to the above-describedstructure. Preferably, a light-emitting region where holes and electronsrecombine is positioned away from the first electrode 101 and the secondelectrode 102 so that quenching due to the proximity of thelight-emitting region and a metal used for electrodes andcarrier-injection layers can be prevented.

Furthermore, in order that the transfer of energy from an excitongenerated in the light-emitting layer can be suppressed, preferably, thehole-transport layer and the electron-transport layer which are incontact with the light-emitting layer 113, particularly acarrier-transport layer in contact with a side closer to therecombination region in the light-emitting layer 113, are formed using asubstance having a wider band gap than the light-emitting substance ofthe light-emitting layer or an emission center substance included in thelight-emitting layer.

Next, an embodiment of a light-emitting element with a structure inwhich a plurality of light-emitting units are stacked (this type oflight-emitting element is also referred to as a stacked or tandemlight-emitting element) is described with reference to FIG. 3C. Thislight-emitting element includes a plurality of light-emitting unitsbetween an anode and a cathode. One light-emitting unit has a structuresimilar to that of the EL layer 103, which is illustrated in FIG. 3A or3B. In other words, the light-emitting element illustrated in FIG. 3A or3B includes a single light-emitting unit, and the light-emitting elementillustrated in FIG. 3C includes a plurality of light-emitting units.

In FIG. 3C, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502, and a charge-generation layer 513 is provided between thefirst light-emitting unit 511 and the second light-emitting unit 512.The first electrode 501 and the second electrode 502 correspond,respectively, to the first electrode 101 and the second electrode 102illustrated in FIG. 3A, and the materials given in the description forFIG. 3A can be used. Furthermore, the first light-emitting unit 511 andthe second light-emitting unit 512 may have the same structure ordifferent structures.

The charge-generation layer 513 has a function of injecting electronsinto one of the light-emitting units and injecting holes into the otherof the light-emitting units when a voltage is applied between the firstelectrode 501 and the second electrode 502. That is, in FIG. 3C, thecharge-generation layer 513 injects electrons into the firstlight-emitting unit 511 and holes into the second light-emitting unit512 when a voltage is applied so that the potential of the firstelectrode becomes higher than the potential of the second electrode.

The charge-generation layer 513 preferably has a structure similar tothe structure of the charge-generation layer 116 described withreference to FIG. 3B. The composite material of an organic compound anda metal oxide has a high carrier-injection property and a highcarrier-transport property; thus, low-voltage driving and low-currentdriving can be achieved. Note that when a surface of a light-emittingunit on the anode side is in contact with the charge-generation layer513, the charge-generation layer 513 can also function as ahole-injection layer in the light-emitting unit and a hole-injectionlayer is not necessarily formed in the light-emitting unit.

In the case where the electron-injection buffer layer 119 is provided,the electron-injection buffer layer functions as the electron-injectionlayer in the light-emitting unit on the anode side and thelight-emitting unit does not further need an electron-injection layer.

The light-emitting element including two light-emitting units isdescribed with reference to FIG. 3C; however, the present invention canbe similarly applied to a light-emitting element in which three or morelight-emitting units are stacked. With a plurality of light-emittingunits partitioned by the charge-generation layer 513 between a pair ofelectrodes as in the light-emitting element according to thisembodiment, it is possible to provide an element which can emit lightwith high luminance with the current density kept low and has a longlifetime. Moreover, a light-emitting device of low power consumption,which can be driven at low voltage, can be achieved.

When light-emitting units have different emission colors, light emissionof a desired color can be obtained as a whole light-emitting element.For example, it is easy to enable a light-emitting element having twolight-emitting units to emit white light as the whole element when theemission colors of the first light-emitting unit are red and green andthe emission color of the second light-emitting unit is blue.

<<Micro Optical Resonator (Microcavity) Structure>>

A light-emitting element with a microcavity structure is formed with theuse of a reflective electrode and a semi-transmissive andsemi-reflective electrode as the pair of electrodes. The reflectiveelectrode and the semi-transmissive and semi-reflective electrodecorrespond to the first electrode and the second electrode describedabove. The light-emitting element with a microcavity structure includesat least an EL layer between the reflective electrode and thesemi-transmissive and semi-reflective electrode. The EL layer includesat least a light-emitting layer functioning as a light-emitting region.

Light emitted from the light-emitting layer included in the EL layer isreflected and resonated by the reflective electrode and thesemi-transmissive and semi-reflective electrode. Note that thereflective electrode has a visible light reflectivity of 40% to 100%,preferably 70% to 100% and a resistivity of 1×10⁻² Ωcm or lower. Inaddition, the semi-transmissive and semi-reflective electrode has avisible light reflectivity of 20% to 80%, preferably 40% to 70%, and aresistivity of 1×10⁻² Ωcm or lower.

In the light-emitting element, by changing the thicknesses of thetransparent conductive film, the composite material, thecarrier-transport material, and the like, the optical path lengthbetween the reflective electrode and the semi-transmissive andsemi-reflective electrode can be changed. Thus, light with a wavelengththat is resonated between the reflective electrode and thesemi-transmissive and semi-reflective electrode can be intensified whilelight with a wavelength that is not resonated therebetween can beattenuated.

Note that light that is emitted from the light-emitting layer andreflected back by the reflective electrode (first reflected light)considerably interferes with light that directly enters thesemi-transmissive and semi-reflective electrode from the light-emittinglayer (first incident light). For this reason, the optical path lengthbetween the reflective electrode and the light-emitting layer ispreferably adjusted to (2n−1)λ/4 (n is a natural number of 1 or largerand λ is a wavelength of a color to be amplified). In that case, thephases of the first reflected light and the first incident light can bealigned with each other and the light emitted from the light-emittinglayer can be further amplified.

Note that in the above structure, the EL layer may be formed of aplurality of light-emitting layers or may be a single light-emittinglayer. The tandem light-emitting element described above may be combinedwith the EL layer; for example, a light-emitting element may have astructure in which a plurality of EL layers is provided, acharge-generation layer is provided between the EL layers, and each ELlayer is formed of a plurality of light-emitting layers or a singlelight-emitting layer.

<<Light-Emitting Device>>

A light-emitting device of one embodiment of the present invention willbe described with reference to FIGS. 6A and 6B. Note that FIG. 6A is atop view of the light-emitting device and FIG. 6B is a cross-sectionalview taken along the lines A-B and C-D in FIG. 6A. The light-emittingdevice includes a driver circuit portion (source line driver circuit)601, a pixel portion 602, and a driver circuit portion (gate line drivercircuit) 603 which are illustrated with dotted lines. Furthermore,reference numeral 604 denotes a sealing substrate and reference numeral605 denotes a sealant. A portion surrounded by the sealant 605 is aspace 607.

Note that a lead wiring 608 is a wiring for transmitting signals to beinput to the source line driver circuit 601 and the gate line drivercircuit 603 and for receiving a video signal, a clock signal, a startsignal, a reset signal, and the like from an FPC (flexible printedcircuit) 609 functioning as an external input terminal. Although onlythe FPC is illustrated here, a printed wiring board (PWB) may beattached to the FPC. The light-emitting device in this specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.6B. The driver circuit portion and the pixel portion are formed over anelement substrate 610. Here, the source line driver circuit 601, whichis the driver circuit portion, and one pixel of the pixel portion 602are illustrated.

In the source line driver circuit 601, a CMOS circuit is formed in whichan n-channel FET 623 and a p-channel FET 624 are combined. The drivercircuit may be formed using various circuits such as a CMOS circuit, aPMOS circuit, or an NMOS circuit. Although a driver-integrated typewhere the driver circuit is formed over the substrate is described inthis embodiment, a driver circuit is not necessarily formed over asubstrate; a driver circuit may be formed outside a substrate.

The pixel portion 602 includes a plurality of pixels including aswitching FET 611, a current controlling FET 612, and a first electrode613 electrically connected to a drain of the current controlling FET612. One embodiment of the present invention is not limited to thisstructure. The pixel portion may include three or more FETs and acapacitor in combination.

The kind and crystallinity of a semiconductor used for the FETs is notparticularly limited; an amorphous semiconductor or a crystallinesemiconductor may be used. Examples of the semiconductor used for theFETs include Group 13 semiconductor, Group 14 semiconductor, compoundsemiconductor, oxide semiconductor, and organic semiconductor materials.Oxide semiconductors are particularly preferable. Examples of the oxidesemiconductor include an In—Ga oxide and an In-M-Zn oxide (M is Al, Ga,Y, Zr, La, Ce, or Nd). Note that an oxide semiconductor material thathas an energy gap of 2 eV or more, preferably 2.5 eV or more, morepreferably 3 eV or more is preferably used, in which case the off-statecurrent of the transistors can be reduced.

Note that an insulator 614 is formed so as to cover an end portion ofthe first electrode 613. The insulator 614 can be formed using apositive photosensitive acrylic resin film here.

In order to improve the coverage, the insulator 614 is formed to have acurved surface with curvature at its upper or lower end portion. Forexample, in the case where a positive photosensitive acrylic resin isused for a material of the insulator 614, only the upper end portion ofthe insulator 614 preferably has a curved surface with a curvatureradius (0.2 μm to 3 μm). Moreover, either a negative photosensitiveresin or a positive photosensitive resin can be used for the insulator614.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. The first electrode 613, the EL layer 616, and the secondelectrode 617 correspond, respectively, to the first electrode 101, theEL layer 103, and the second electrode 102 in FIG. 3A or 3B, andcorrespond, respectively, to the first electrode 501, the EL layer (511to 513), and the second electrode 502 in FIG. 3C.

The EL layer 616 preferably contains an organometallic complex. Theorganometallic complex is preferably used as an emission centersubstance in the light-emitting layer.

The sealing substrate 604 is attached using the sealant 605 to theelement substrate 610; thus, a light-emitting element 618 is provided inthe space 607 surrounded by the element substrate 610, the sealingsubstrate 604, and the sealant 605. The space 607 is filled with filler,and may be filled with an inert gas (e.g., nitrogen or argon), thesealant 605, or the like. It is preferable that the sealing substrate beprovided with a recessed portion and a drying agent be provided in therecessed portion, in which case deterioration due to influence ofmoisture can be suppressed.

An epoxy-based resin or glass frit is preferably used for the sealant605. A material used for them is desirably a material which does nottransmit moisture or oxygen as much as possible. As the elementsubstrate 610 and the sealing substrate 604, for example, a glasssubstrate, a quartz substrate, or a plastic substrate formed of fiberreinforced plastic (FRP), polyvinyl fluoride (PVF), polyester, oracrylic can be used.

Note that in this specification and the like, a transistor or alight-emitting element can be formed using any of a variety ofsubstrates, for example. The type of a substrate is not limited to acertain type. As the substrate, a semiconductor substrate (e.g., asingle crystal substrate or a silicon substrate), an SOI substrate, aglass substrate, a quartz substrate, a plastic substrate, a metalsubstrate, a stainless steel substrate, a substrate including stainlesssteel foil, a tungsten substrate, a substrate including tungsten foil, aflexible substrate, an attachment film, paper including a fibrousmaterial, a base material film, or the like can be used, for example. Asan example of a glass substrate, a barium borosilicate glass substrate,an aluminoborosilicate glass substrate, a soda lime glass substrate, orthe like can be given. Examples of the flexible substrate, theattachment film, the base material film, or the like are as follows:plastic typified by polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), and polyether sulfone (PES). Another example is asynthetic resin such as acrylic. Alternatively, polytetrafluoroethylene(PTFE), polypropylene, polyester, polyvinyl fluoride, polyvinylchloride, or the like can be used. Alternatively, polyamide, polyimide,aramid, epoxy, an inorganic film formed by evaporation, paper, or thelike can be used. Specifically, the use of semiconductor substrates,single crystal substrates, SOI substrates, or the like enables themanufacture of small-sized transistors with a small variation incharacteristics, size, shape, or the like and with high currentcapability. A circuit using such transistors achieves lower powerconsumption of the circuit or higher integration of the circuit.

Alternatively, a flexible substrate may be used as the substrate, andthe transistor or the light-emitting element may be provided directlyover the flexible substrate. Still alternatively, a separation layer maybe provided between a substrate and the transistor or between thesubstrate and the light-emitting element. The separation layer can beused when part or the whole of a semiconductor device formed over theseparation layer is separated from the substrate and transferred ontoanother substrate. In such a case, the transistor can be transferred toa substrate having low heat resistance or a flexible substrate as well.For the above separation layer, a stack including inorganic films, whichare a tungsten film and a silicon oxide film, or an organic resin filmof polyimide or the like formed over a substrate can be used, forexample.

In other words, a transistor or a light-emitting element may be formedusing one substrate, and then transferred to another substrate. Examplesof the substrate to which the transistor or the light-emitting elementis transferred include, in addition to the above-described substratesover which transistors can be formed, a paper substrate, a cellophanesubstrate, an aramid film substrate, a polyimide film substrate, a stonesubstrate, a wood substrate, a cloth substrate (including a naturalfiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon,polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra,rayon, or regenerated polyester), or the like), a leather substrate, anda rubber substrate. When such a substrate is used, a transistor withexcellent properties or a transistor with low power consumption can beformed, a device with high durability and high heat resistance can beprovided, or a reduction in weight or thickness can be achieved.

FIGS. 7A and 7B each illustrate an example of a light-emitting device inwhich full color display is achieved by forming a light-emitting elementexhibiting white light emission and using coloring layers (colorfilters) and the like. In FIG. 7A, a substrate 1001, a base insulatingfilm 1002, a gate insulating film 1003, gate electrodes 1006, 1007, and1008, a first interlayer insulating film 1020, a second interlayerinsulating film 1021, a peripheral portion 1042, a pixel portion 1040, adriver circuit portion 1041, first electrodes 1024W, 1024R, 1024G, and1024B of light-emitting elements, a partition 1025, an EL layer 1028, asecond electrode 1029 of the light-emitting elements, a sealingsubstrate 1031, a sealant 1032, and the like are illustrated.

In FIG. 7A, coloring layers (a red coloring layer 1034R, a greencoloring layer 1034G, and a blue coloring layer 1034B) are provided on atransparent base material 1033. A black layer (a black matrix) 1035 maybe additionally provided. The transparent base material 1033 providedwith the coloring layers and the black layer is positioned and fixed tothe substrate 1001. Note that the coloring layers and the black layerare covered with an overcoat layer.

In FIG. 7A, light emitted from some of the light-emitting layers doesnot pass through the coloring layers, while light emitted from theothers of the light-emitting layers passes through the coloring layers.Since light which does not pass through the coloring layers is white andlight which passes through any one of the coloring layers is red, blue,or green, an image can be displayed using pixels of the four colors.

FIG. 7B illustrates an example in which the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) are formed between the gate insulating film 1003and the first interlayer insulating film 1020. As in this structure, thecoloring layers may be provided between the substrate 1001 and thesealing substrate 1031.

The above-described light-emitting device has a structure in which lightis extracted from the substrate 1001 side where the FETs are formed (abottom emission structure), but may have a structure in which light isextracted from the sealing substrate 1031 side (a top emissionstructure). FIG. 8 is a cross-sectional view of a light-emitting devicehaving a top emission structure. In that case, a substrate which doesnot transmit light can be used as the substrate 1001. The process up tothe step of forming of a connection electrode which connects the FET andthe anode of the light-emitting element is performed in a manner similarto that of the light-emitting device having a bottom emission structure.Then, a third interlayer insulating film 1037 is formed to cover anelectrode 1022. This insulating film may have a planarization function.The third interlayer insulating film 1037 can be formed using a materialsimilar to that of the second interlayer insulating film, or can beformed using any other various materials.

The first electrodes 1024W, 1024R, 1024G, and 1024B of thelight-emitting elements each function as an anode here, but may functionas a cathode. Furthermore, in the case of the light-emitting devicehaving a top emission structure as illustrated in FIG. 8, the firstelectrodes are preferably reflective electrodes. The EL layer 1028 isformed to have a structure similar to the structure of the EL layer 103in FIG. 3A or 3B or the EL layer (511 to 513) in FIG. 3C, with whichwhite light emission can be obtained.

In the case of a top emission structure as illustrated in FIG. 8,sealing can be performed with the sealing substrate 1031 on which thecoloring layers (the red coloring layer 1034R, the green coloring layer1034G, and the blue coloring layer 1034B) are provided. The sealingsubstrate 1031 may be provided with the black layer (the black matrix)1035 which is positioned between pixels. The coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) and the black layer may be covered with theovercoat layer. Note that a light-transmitting substrate is used as thesealing substrate 1031.

Although an example in which full color display is performed using fourcolors of red, green, blue, and white is shown here, there is noparticular limitation and full color display using three colors of red,green, and blue or four colors of red, green, blue, and yellow may beperformed.

FIGS. 9A and 9B illustrate a passive matrix light-emitting device of oneembodiment of the present invention. FIG. 9A is a perspective view of alight-emitting device, and FIG. 9B is a cross-sectional view taken alongthe line X-Y of FIG. 9A. In FIGS. 9A and 9B, an EL layer 955 is providedbetween an electrode 952 and an electrode 956 over a substrate 951. Anend portion of the electrode 952 is covered with an insulating layer953. A partition layer 954 is provided over the insulating layer 953.Sidewalls of the partition layer 954 are aslope such that the distancebetween the sidewalls is gradually narrowed toward the surface of thesubstrate. That is, a cross section in a short side direction of thepartition layer 954 is a trapezoidal shape, and a lower side (the sidefacing the same direction as the plane direction of the insulating layer953 and touching the insulating layer 953) is shorter than an upper side(the side facing the same direction as the plane direction of theinsulating layer 953, and not touching the insulating layer 953). Byproviding the partition layer 954 in this manner, defects of thelight-emitting element due to static charge and the like can beprevented.

Since many minute light-emitting elements arranged in a matrix can becontrolled with the FETs formed in the pixel portion, theabove-described light-emitting device can be suitably used as a displaydevice for displaying images.

<<Lighting Device>>

A lighting device of one embodiment of the present invention isdescribed with reference to FIGS. 10A and 10B. FIG. 10B is a top view ofthe lighting device, and FIG. 10A is a cross-sectional view taken alongthe line e-f in FIG. 10B.

In the lighting device, a first electrode 401 is formed over a substrate400 which is a support and has a light-transmitting property. The firstelectrode 401 corresponds to the first electrode 101 in FIGS. 3A and 3B.When light is extracted through the first electrode 401 side, the firstelectrode 401 is formed using a material having a light-transmittingproperty.

A pad 412 for applying voltage to a second electrode 404 is providedover the substrate 400.

An EL layer 403 is formed over the first electrode 401. The EL layer 403corresponds to, for example, the EL layer 103 in FIGS. 3A and 3B or theEL layer (511 to 513) in FIG. 3C. For these structures, thecorresponding description can be referred to.

The second electrode 404 is formed to cover the EL layer 403. The secondelectrode 404 corresponds to the second electrode 102 in FIG. 3A or 3B.The second electrode 404 contains a material having high reflectivitywhen light is extracted through the first electrode 401 side. The secondelectrode 404 is connected to the pad 412, whereby voltage is appliedthereto.

A light-emitting element is formed with the first electrode 401, the ELlayer 403, and the second electrode 404. The light-emitting element isfixed to a sealing substrate 407 with sealants 405 and 406 and sealingis performed, whereby the lighting device is completed. It is possibleto use only either the sealant 405 or the sealant 406. In addition, theinner sealant 406 (not illustrated in FIG. 10B) can be mixed with adesiccant that enables moisture to be adsorbed, which results inimproved reliability.

When part of the pad 412 and part of the first electrode 401 areextended to the outside of the sealants 405 and 406, the extended partscan function as external input terminals. An IC chip 420 mounted with aconverter or the like may be provided over the external input terminals.

<<Electronic Device>>

Examples of an electronic device of one embodiment of the presentinvention are described. Examples of the electronic device include atelevision device (also referred to as a television or a televisionreceiver), a monitor of a computer or the like, a camera such as adigital camera or a digital video camera, a digital photo frame, amobile phone (also referred to as a mobile telephone or a mobile phonedevice), a portable game console, a portable information terminal, anaudio reproducing device, and a large-sized game machine such as apachinko machine. Specific examples of these electronic devices aredescribed below.

FIG. 11A illustrates an example of a television device. In thetelevision device, a display portion 7103 is incorporated in a housing7101. In addition, here, the housing 7101 is supported by a stand 7105.Images can be displayed on the display portion 7103, and in the displayportion 7103, light-emitting elements are arranged in a matrix.

The television device can be operated with an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device is provided with a receiver, a modem,and the like. With the use of the receiver, a general televisionbroadcast can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) data communication can beperformed.

FIG. 11B1 illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured by using light-emitting elements arrangedin a matrix in the display portion 7203. The computer illustrated inFIG. 11B1 may have a structure illustrated in FIG. 11B2. The computerillustrated in FIG. 11B2 is provided with a second display portion 7210instead of the keyboard 7204 and the pointing device 7206. The seconddisplay portion 7210 is a touch panel, and input can be performed byoperation of display for input on the second display portion 7210 with afinger or a dedicated pen. The second display portion 7210 can alsodisplay images other than the display for input. The display portion7203 may also be a touch panel. Connecting the two screens with a hingecan prevent troubles; for example, the screens can be prevented frombeing cracked or broken while the computer is being stored or carried.

FIGS. 11C and 11D illustrate an example of a portable informationterminal. The portable information terminal is provided with a displayportion 7402 incorporated in a housing 7401, operation buttons 7403, anexternal connection port 7404, a speaker 7405, a microphone 7406, andthe like. Note that the portable information terminal has the displayportion 7402 including light-emitting elements arranged in a matrix.

Information can be input to the portable information terminalillustrated in FIGS. 11C and 11D by touching the display portion 7402with a finger or the like. In that case, operations such as making acall and creating e-mail can be performed by touching the displayportion 7402 with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or creating e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be input. In that case, itis preferable to display a keyboard or number buttons on almost theentire screen of the display portion 7402.

When a detection device including a sensor such as a gyroscope sensor oran acceleration sensor for sensing inclination is provided inside theportable information terminal, screen display of the display portion7402 can be automatically changed by determining the orientation of themobile phone (whether the mobile phone is placed horizontally orvertically).

The screen modes are switched by touching the display portion 7402 oroperating the operation buttons 7403 of the housing 7401. Alternatively,the screen modes can be switched depending on kinds of images displayedon the display portion 7402. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is switched to the display mode. When the signal is a signalof text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed within a specified period while a signal sensed byan optical sensor in the display portion 7402 is sensed, the screen modemay be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenby touch on the display portion 7402 with the palm or the finger,whereby personal authentication can be performed. Furthermore, byproviding a backlight or a sensing light source which emitsnear-infrared light in the display portion, an image of a finger vein, apalm vein, or the like can be taken.

Note that in the above electronic devices, any of the structuresdescribed in this specification can be combined as appropriate.

The display portion preferably includes a light-emitting element of oneembodiment of the present invention. The light-emitting element can havehigh emission efficiency. In addition, the light-emitting element can bedriven with low drive voltage. Thus, the electronic device including thelight-emitting element of one embodiment of the present invention canhave low power consumption.

FIG. 12 illustrates an example of a liquid crystal display deviceincluding the light-emitting element for a backlight. The liquid crystaldisplay device illustrated in FIG. 12 includes a housing 901, a liquidcrystal layer 902, a backlight unit 903, and a housing 904. The liquidcrystal layer 902 is connected to a driver IC 905. The light-emittingelement is used for the backlight unit 903, to which current is suppliedthrough a terminal 906.

As the light-emitting element, a light-emitting element of oneembodiment of the present invention is preferably used. By including thelight-emitting element, the backlight of the liquid crystal displaydevice can have low power consumption.

FIG. 13 illustrates an example of a desk lamp of one embodiment of thepresent invention. The desk lamp illustrated in FIG. 13 includes ahousing 2001 and a light source 2002, and a lighting device including alight-emitting element is used as the light source 2002.

FIG. 14 illustrates an example of an indoor lighting device 3001. Thelight-emitting element of one embodiment of the present invention ispreferably used in the lighting device 3001.

An automobile of one embodiment of the present invention is illustratedin FIG. 15. In the automobile, light-emitting elements are used for awindshield and a dashboard. Display regions 5000 to 5005 are preferablyformed by using the light-emitting elements of one embodiment of thepresent invention. This suppresses power consumption of the displayregions 5000 to 5005, showing suitability for use in an automobile.

The display regions 5000 and 5001 are display devices which are providedin the automobile windshield and which include the light-emittingelements. When a first electrode and a second electrode are formed ofelectrodes having light-transmitting properties in these light-emittingelements, what is called a see-through display device, through which theopposite side can be seen, can be obtained. Such see-through displaydevices can be provided even in the windshield of the automobile,without hindering the vision. Note that in the case where a transistorfor driving the light-emitting element is provided, a transistor havinga light-transmitting property, such as an organic transistor using anorganic semiconductor material or a transistor using an oxidesemiconductor, is preferably used.

The display region 5002 is a display device which is provided in apillar portion and which includes the light-emitting element. Thedisplay region 5002 can compensate for the view hindered by the pillarportion by showing an image taken by an imaging unit provided in the carbody. Similarly, the display region 5003 provided in the dashboard cancompensate for the view hindered by the car body by showing an imagetaken by an imaging unit provided in the outside of the car body, whichleads to elimination of blind areas and enhancement of safety. Showingan image so as to compensate for the area which a driver cannot seemakes it possible for the driver to confirm safety easily andcomfortably.

The display region 5004 and the display region 5005 can provide avariety of kinds of information such as navigation information, aspeedometer, a tachometer, a mileage, a fuel meter, a gearshiftindicator, and air-condition setting. The content or layout of thedisplay can be changed freely by a user as appropriate. Note that suchinformation can also be shown by the display regions 5000 to 5003. Thedisplay regions 5000 to 5005 can also be used as lighting devices.

FIGS. 16A and 16B illustrate an example of a foldable tablet terminal.In FIG. 16A, the tablet terminal is opened, and includes a housing 9630,a display portion 9631 a, a display portion 9631 b, a switch 9034 forswitching display modes, a power switch 9035, a switch 9036 forswitching to power-saving mode, a fastener 9033, and an operation switch9038. Note that in the tablet terminal, one or both of the displayportion 9631 a and the display portion 9631 b are formed using alight-emitting device which includes the light-emitting element of oneembodiment of the present invention.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9637 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region has a touchpanel function is illustrated as an example, the structure of thedisplay portion 9631 a is not limited thereto. The whole region in thedisplay portion 9631 a may have a touch panel function. For example, thedisplay portion 9631 a can display keyboard buttons in the whole regionto be a touch panel, and the display portion 9631 b can be used as adisplay screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a switching button 9639 forshowing/hiding a keyboard on the touch panel is touched with a finger, astylus, or the like, the keyboard can be displayed on the displayportion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The switch 9034 for switching display modes can switch the displaybetween portrait mode, landscape mode, and the like, and betweenmonochrome display and color display, for example. The switch 9036 forswitching to power-saving mode can control display luminance to beoptimal in accordance with the amount of external light in use of thetablet terminal which is sensed by an optical sensor incorporated in thetablet terminal. Another sensing device including a sensor for sensinginclination, such as a gyroscope sensor or an acceleration sensor, maybe incorporated in the tablet terminal, in addition to the opticalsensor.

Note that FIG. 16A illustrates an example in which the display portion9631 a and the display portion 9631 b have the same display area;however, without limitation thereon, one of the display portions may bedifferent from the other display portion in size and display quality.For example, one display panel may be capable of higher-definitiondisplay than the other display panel.

FIG. 16B illustrates the tablet terminal which is folded. The tabletterminal in this embodiment includes the housing 9630, a solar cell9633, a charge and discharge control circuit 9634, a battery 9635, and aDCDC converter 9636. Note that in FIG. 16B, an example in which thecharge and discharge control circuit 9634 includes the battery 9635 andthe DCDC converter 9636 is illustrated.

Since the tablet terminal can be folded, the housing 9630 can be closedwhen the tablet terminal is not used. As a result, the display portion9631 a and the display portion 9631 b can be protected; thus, a tabletterminal which has excellent durability and excellent reliability interms of long-term use can be provided.

In addition, the tablet terminal illustrated in FIGS. 16A and 16B canhave a function of displaying a variety of kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touch panel, the display portion, a video signalprocessing portion, or the like. Note that the solar cell 9633 ispreferably provided on one or two surfaces of the housing 9630, in whichcase the battery 9635 can be charged efficiently.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 16B are described with reference to ablock diagram in FIG. 16C. FIG. 16C illustrates the solar cell 9633, thebattery 9635, the DCDC converter 9636, a converter 9638, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DCDC converter9636, the converter 9638, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634 illustrated in FIG. 16B.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell is raised or lowered by the DCDCconverter 9636 so that the power has a voltage for charging the battery9635. Then, when power supplied from the battery 9635 charged by thesolar cell 9633 is used for the operation of the display portion 9631,the switch SW1 is turned on and the voltage of the power is raised orlowered by the converter 9638 so as to be voltage needed for the displayportion 9631. In addition, when display on the display portion 9631 isnot performed, the switch SW1 is turned off and the switch SW2 is turnedon so that charge of the battery 9635 may be performed.

Although the solar cell 9633 is described as an example of a powergeneration means, the power generation means is not particularlylimited, and the battery 9635 may be charged by another power generationmeans such as a piezoelectric element or a thermoelectric conversionelement (Peltier element). The battery 9635 may be charged by anon-contact power transmission module capable of performing charging bytransmitting and receiving power wirelessly (without contact), or any ofthe other charge means used in combination, and the power generationmeans is not necessarily provided.

One embodiment of the present invention is not limited to the tabletterminal having the shape illustrated in FIGS. 16A to 16C as long as thedisplay portion 9631 is included.

FIGS. 17A to 17C illustrate a foldable portable information terminal9310. FIG. 17A illustrates the portable information terminal 9310 whichis opened. FIG. 17B illustrates the portable information terminal 9310which is being opened or being folded. FIG. 17C illustrates the portableinformation terminal 9310 which is folded. The portable informationterminal 9310 is highly portable when folded. The portable informationterminal 9310 is highly browsable when opened because of a seamlesslarge display region.

A display panel 9311 is supported by three housings 9315 joined togetherby hinges 9313. Note that the display panel 9311 may be a touch panel(an input/output device) including a touch sensor (an input device). Bybending the display panel 9311 at a connection portion between twohousings 9315 with the use of the hinges 9313, the portable informationterminal 9310 can be reversibly changed in shape from an opened state toa folded state. A light-emitting device of one embodiment of the presentinvention can be used for the display panel 9311. A display region 9312in the display panel 9311 is a display region that is positioned at aside surface of the portable information terminal 9310 that is folded.On the display region 9312, information icons, file shortcuts offrequently used applications or programs, and the like can be displayed,and confirmation of information and start of application can be smoothlyperformed.

Example 1

In this example, results of calculating the parameter a of alight-emitting element (Light-emitting Element 1) of one embodiment ofthe present invention with high efficiency are described in detail. Forthe above purpose, a light-emitting element (Light-emitting Element 1-1)for measurement which includes a light-emitting layer having the samestructure as that of Light-emitting Element 1 and in which the luminancein the front direction is reduced as much as possible was alsofabricated. FIG. 18 illustrates the structure of the light-emittingelement.

First, a fabrication method and the structure of the light-emittingelement of one embodiment of the present invention are described.Organic compounds used in the light-emitting element of one embodimentof the present invention are given below.

(Fabrication Method of Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness of the first electrode 101 was70 nm and the electrode area was 2 mm×2 mm.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation to a thickness of 10 nm at a weightratio of 4:2 (=DBT3P-II: molybdenum oxide) by an evaporation methodusing resistance heating, so that the hole-injection layer 111 wasformed.

Next, a film of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) represented by Structural Formula (ii) was formedby evaporation to a thickness of 30 nm on the hole-injection layer 111to form the hole-transport layer 112.

After that, the light-emitting layer 113 was formed by co-evaporation of7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) represented by Structural Formula (iii) andN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) represented by Structural Formula (iv) ata weight ratio of 1:0.03 (=cgDBCzPA: 1,6mMemFLPAPrn) to a thickness of15 nm. In the step of forming the light-emitting layer 113 (at the timeof evaporation), the total pressure was approximately 1×10⁻⁴ Pa and thepartial pressure of carbon dioxide whose molecular weight was detectedas 44 was approximately 6×10⁻⁷ Pa when the measurement was performed byQ-mass provided in an evaporation chamber. That is, the percentage ofthe partial pressure of carbon dioxide with respect to the totalpressure at the time of evaporation was approximately 0.6%. In thismanner, it is important that the percentage of the partial pressure ofcarbon dioxide with respect to the total pressure at the time ofevaporation is higher than 0.03%, specifically, higher than or equal to0.1%. Since only gases whose molecular weight is 1 to 200 can bedetected by Q-mass, the total pressure measured by Q-mass is, strictlyspeaking, different from a practical total pressure in the chamber.However, the partial pressure of gas components whose molecular weightexceeds 200 is negligible; thus, with the use of either the totalpressure measured by Q-mass or the practical total pressure in thechamber, similar results can be obtained.

Then, on the light-emitting layer 113, cgDBCzPA was deposited byevaporation to a thickness of 20 nm, and bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (v) wasdeposited by evaporation to a thickness of 15 nm to form theelectron-transport layer 114.

After the formation of the electron-transport layer 114, lithium oxide(Li₂O) was deposited by evaporation to a thickness of 0.1 nm to form theelectron-injection layer 115. Then, aluminum was deposited byevaporation to a thickness of 200 nm to form the second electrode 102.Through the above-described steps, Light-emitting Element 1 of thisexample was fabricated.

(Fabrication Method of Light-Emitting Element 1-1)

Light-emitting Element 1-1 was fabricated in the following manner. Afterthe electron-injection layer 115 of Light-emitting Element 1 was formed,copper phthalocyanine (abbreviation: CuPc) represented by StructuralFormula (vi) was deposited by evaporation to a thickness of 2 nm to formthe electron-relay layer 118, and then, DBT3P-II and molybdenum(VI)oxide were deposited by co-evaporation at a weight ratio of 2:1(=DBT3P-II: molybdenum oxide) to a thickness of 60 nm to form the p-typelayer 117. In such a manner, a layer for adjusting the thickness wasformed.

The element structures of Light-emitting Elements 1 and 1-1 are shown inthe following table.

TABLE 1 hole-injection hole-transport light-emitting electron-injectionlayer for adjusting layer layer layer electron-transport layer layer thethickness 10 nm 30 nm 15 nm 20 nm 15 nm 0.1 nm 2 nm 60 nm Element 1DBT3P-II:MoOx BPAFLP cgDBCzPA: cgDBCzPA BPhen Li₂O — — (4:2)1,6mMemFLPAPrn Element 1-1 (1:0.03) CuPc DBT3P-II:MoOx (4:2)

Light-emitting Elements 1 and 1-1 were each sealed using a glasssubstrate in a glove box containing a nitrogen atmosphere so as not tobe exposed to the air (specifically, a sealant was applied to surroundthe element and UV treatment and heat treatment at 80° C. for 1 hourwere performed at the time of sealing). Then, the initialcharacteristics of Light-emitting Elements 1 and 1-1 were measured. Notethat the measurement was carried out in an atmosphere kept at 25° C.

FIG. 19 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 1. Table 2 shows the main characteristics ofLight-emitting Elements 1 and 1-1 at a luminance of about 1000 cd/m².

TABLE 2 External Current Current Quantum Voltage Current densitychromaticity Efficiency Efficiency (V) (mA) (mA/cm²) x y (cd/A) (%)Element 1 3.3 0.27 6.9 0.14 0.18 14 11 Element 1-1 4.6 4.38 110 0.230.36 1.0 0.4

As shown in Table 2, Light-emitting Element 1 has a very highefficiency, e.g., an external quantum efficiency of 11%.

Light-emitting Element 1-1 includes the layer for adjusting thethickness in addition to the components of Light-emitting Element 1. Byadjusting the optical path length, light that travels in the frontdirection is attenuated, so that a representing the orientation statecan be easily obtained. A difference between the structure and thefabrication method of Light-emitting Element 1 and those ofLight-emitting Element 1-1 lies only in the layer for adjusting thethickness; thus, the orientation states of light-emitting substances inthe light-emitting layers are assumed to be the same.

The orientation state of a light-emitting material in the light-emittinglayer was examined with the use of Light-emitting Element 1-1. First,the angle dependence of the shape of the EL emission spectrum wasmeasured by measuring the EL spectrum in steps of 1° in such a mannerthat, as shown in FIG. 20, the substrate provided with Light-emittingElement 1-1 was inclined to a detector (a photonic multichannel analyzerPMA-12, produced by Hamamatsu Photonics K.K.) from θ=0° to 80°. In thismeasurement, a linear polarizer (Glan-Taylor prism) was disposed betweenLight-emitting Element 1-1 and the detector to be perpendicular to thesubstrate surface in order to remove an S polarization from lightemitted from Light-emitting Element 1-1, so that the spectrum of only aP polarization was measured.

FIG. 21 is a graph in which the vertical axis represents the integratedintensity of the EL emission spectrum from 440 nm to 956 nm depending onthe angle (θ) and the horizontal axis represents the angle (θ) of thedetector. In FIG. 21, a curve plotted by open squares represents themeasured values, and a solid curve and a dashed curve represent thecalculation results obtained by setfos which is an organic devicesimulator. The calculation was performed by inputting the thickness ofeach layer in the element, the measured values of the refractive indexand the extinction efficiency, the measured value of the emissionspectrum of a dopant, the position and the width of a light-emittingregion, and the orientation parameter a. Among them, the thickness ofeach layer, the refractive index, and the extinction efficiency weremeasured with the use of a spectroscopic ellipsometer (M-2000U, producedby J.A. Woollam Japan Corp.). For the measurement, a film was used whichwas formed of the light-emitting material over a quartz substrate by avacuum evaporation method to a thickness of 150 nm. The emissionspectrum of the dopant was measured using a fluorescencespectrophotometer (FS920, produced by Hamamatsu Photonics K.K.). For themeasurement, a film was used which was formed by co-evaporation ofcgDBCzPA and 1,6mMemFLPAPrn using a vacuum evaporation method at aweight ratio of 1:0.03 (=cgDBCzPA: 1,6mMemFLPAPrn) over a quartzsubstrate so that cgDBCzPA has a thickness of 50 nm. In the calculationby setfos, further, the light-emitting region was set. Thelight-emitting region was assumed to spread such that the recombinationprobability was attenuated exponentially in the cathode direction withthe interface between the hole-transport layer and the light-emittinglayer as a top, specifically, the recombination probability wasattenuated to 1/e at a thickness of 10 nm. Thus, the angle dependence ofthe integrated intensity of the emission spectrum corresponding to eachparameter a can be calculated. In Light-emitting Element 1-1, themeasured value well fitted a curve of a=0.16.

FIG. 22 is a 2D contour map obtained by measuring the angle dependenceof the EL emission spectrum of Light-emitting Element 1-1. FIG. 23 is a2D contour map obtained by calculation. FIG. 22 and FIG. 23 show thatthese 2D contour maps match well. This indicates that the orientation ofthe light-emitting materials in Light-emitting Elements 1 and 1-1 wasaccurately obtained in both the experiment and the calculation.

The parameter a is ⅓≈0.33 in the case where the transition dipole isoriented in a random direction, and the parameter a is 0 in the casewhere the transition dipole is completely parallel to the substrate. Thelight extraction efficiency in the case of a=0 is 1.5 times the lightextraction efficiency in the case of a=⅓≈0.33. In view of this, thelight-emitting element of this example in which a=0.16 has lightextraction efficiency that is 1.26 times the light extraction efficiencyof the light-emitting element with a random orientation. That is, thelight-emitting element of one embodiment of the present invention has anemission efficiency that is 1.26 times the emission efficiency of thelight-emitting element with a random orientation.

Light-emitting Elements 1 and 1-1 are formed using the same material fortheir light-emitting layers by the same method; thus, it can be saidthat Light-emitting Element 1 has, like Light-emitting Element 1-1, anorientation of a=0.16. Light-emitting Element 1 has a very high externalquantum efficiency of 11%. It is found that a light-emitting elementwith high emission efficiency can be obtained by setting a to less thanor equal to 0.2. Measurement results of transient EL emission indicatethat TTA also occurred in this element. The quantum yield of the filmwas 0.85 on average in the case of excitation light at 360 nm; the filmwas formed by co-evaporation of cgDBCzPA and 1,6mMemFLPAPrn using avacuum evaporation method at a weight ratio of 1:0.03 (=cgDBCzPA:1,6mMemFLPAPrn) so that cgDBCzPA has a thickness of 50 nm. That is, thelight-emitting layer of this example has a parameter a of 0.2 or lessand a fluorescent quantum yield of 0.84 or higher; thus, even when TTAdoes not occur, this layer satisfies theoretical conditions under whichthe external quantum efficiency can be higher than or equal to 7.5%. Inaddition to the above, since TTA occurs, a light-emitting element whichhas excellent characteristics, an external quantum efficiency of higherthan 10%, was able to be obtained in this example.

Example 2

In this example, results of calculating the parameter a of alight-emitting element (Light-emitting Element 2) of one embodiment ofthe present invention with high efficiency are described in detail. Forthe above purpose, a light-emitting element (Light-emitting Element 2-1)for measurement which includes a light-emitting layer having the samestructure as that of Light-emitting Element 2 and in which the luminancein the front direction is reduced as much as possible was alsofabricated.

First, a fabrication method and the structure of the light-emittingelement of one embodiment of the present invention are described.Organic compounds used in the light-emitting element of one embodimentof the present invention are given below.

(Fabrication Method of Light-Emitting Element 2)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness of the first electrode 101 was70 nm and the electrode area was 2 mm×2 mm.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation to a thickness of 50 nm at a weightratio of 4:2 (=DBT3P-II: molybdenum oxide) by an evaporation methodusing resistance heating, so that the hole-injection layer 111 wasformed.

Next, a film of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) represented by Structural Formula (ii) was formedby evaporation to a thickness of 20 nm on the hole-injection layer 111to form the hole-transport layer 112.

After that, the light-emitting layer 113 was formed in the followingmanner: 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[/h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (vii),N-(1,1′-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9H-fluoren-2-amine (abbreviation: PCBBiF) represented by Structural Formula(viii), andbis{2-[5-methyl-6-(2-methylphenyl)-4-pyrimidinyl-κN3]phenyl-κC}(2,4-pentanedionato-κ²O,O′)iridium(III))(abbreviation: [Ir(mpmppm)₂(acac)]) represented by Structural Formula(ix) were deposited by co-evaporation at a weight ratio of 0.8:0.2:0.05(=2mDBTBPDBq-II:PCBBiF:[Ir(mpmppm)₂(acac)]) to a thickness of 40 nm. Inthe step of forming the light-emitting layer 113 (at the time ofevaporation), the total pressure was approximately 2×10⁻⁴ Pa and thepartial pressure of carbon dioxide whose molecular weight was detectedas 44 was approximately 1×10⁻⁶ Pa when the measurement was performed byQ-mass provided in an evaporation chamber. That is, the percentage ofthe partial pressure of carbon dioxide with respect to the totalpressure at the time of evaporation was approximately 0.5%. In thismanner, it is important that the percentage of the partial pressure ofcarbon dioxide with respect to the total pressure at the time ofevaporation is higher than 0.03%, specifically, higher than or equal to0.1%. Since only gases whose molecular weight is 1 to 200 can bedetected by Q-mass, the total pressure measured by Q-mass is, strictlyspeaking, different from a practical total pressure in the chamber. Thepartial pressure of gas components whose molecular weight exceeds 200 isnegligible; thus, with the use of either the total pressure measured byQ-mass or the practical total pressure in the chamber, similar resultscan be obtained.

Then, on the light-emitting layer 113, 2mDBTBPDBq-II was deposited byevaporation to a thickness of 25 nm, and bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (v) wasdeposited by evaporation to a thickness of 10 nm to form theelectron-transport layer 114.

After the formation of the electron-transport layer 114, lithium oxide(Li₂O) was deposited by evaporation to a thickness of 0.1 nm to form theelectron-injection layer 115. Then, aluminum was deposited byevaporation to a thickness of 200 nm to form the second electrode 102.Through the above-described steps, Light-emitting Element 2 of thisexample was fabricated.

(Fabrication Method of Light-Emitting Element 2-1)

Light-emitting Element 2-1 was fabricated in the following manner. Afterthe electron-injection layer 115 of Light-emitting Element 2 was formed,copper phthalocyanine (abbreviation: CuPc) represented by StructuralFormula (vi) was deposited by evaporation to a thickness of 2 nm, andthen, DBT3P-II and molybdenum(VI) oxide were deposited by co-evaporationat a weight ratio of 2:1 (=DBT3P-II: molybdenum oxide) to a thickness of85 nm. In such a manner, a layer for adjusting the thickness was formed.

The element structures of Light-emitting elements 2 and 2-1 are shown inthe following table.

TABLE 3 hole-injection hole-transport light-emitting electron-injectionlayer for adjusting layer layer layer electron-transport layer layer thethickness 50 nm 20 nm 40 nm 25 nm 10 nm 0.1 nm 2 nm 85 nm Element 2DBT3P-II:MoOx BPAFLP 2mDBTBPDBq-II: 2mDBTBPDBq-II BPhen Li₂O — — (4:2)PCBBiF: Element 2-1 [Ir(mpmppm)₂(acac)] CuPc DBT3P-II:MoOx(0.8:0.2:0.05) (4:2)

Light-emitting Elements 2 and 2-1 were each sealed using a glasssubstrate in a glove box containing a nitrogen atmosphere so as not tobe exposed to the air (specifically, a sealant was applied to surroundthe element and UV treatment and heat treatment at 80° C. for 1 hourwere performed at the time of sealing). Then, the initialcharacteristics of Light-emitting Elements 2 and 2-1 were measured. Notethat the measurement was carried out in an atmosphere kept at 25° C.

FIG. 24 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 2. Table 4 shows the main characteristics ofLight-emitting Elements 2 and 2-1 at a luminance of about 1000 cd/m².

TABLE 4 External Current Current Quantum Voltage Current densitychromaticity Efficiency Efficiency (V) (mA) (mA/cm²) x y (cd/A) (%)Element 2 2.8 0.03 0.8 0.49 0.50 105 30 Element 2-1 4.2 0.90 23 0.490.50 4.1 2.5

As shown in Table 4, Light-emitting Element 2 has a very highefficiency, e.g., an external quantum efficiency of 30%. Thelight-emitting material used in Light-emitting Element 2,[Ir(mpmppm)₂(acac)], has an emission quantum yield (ϕ) of 0.84. Assumingthat the carrier balance (γ) is 1 and the proportion of generatedexcitons (α) is 1, the light extraction efficiency (χ) is calculated to35.7%, which is much higher than a general theoretical light extractionefficiency of 20% to 30%.

Light-emitting Element 2-1 includes the layer for adjusting thethickness in addition to the components of Light-emitting Element 2. Byadjusting the optical path length, light that travels in the frontdirection is attenuated, so that a representing the orientation statecan be easily obtained. A difference between the structure and thefabrication method of Light-emitting Element 2 and those ofLight-emitting Element 2-1 lies only in the layer for adjusting thethickness; thus, the orientation states of light-emitting substances inthe light-emitting layers are assumed to be the same.

The orientation state of a light-emitting material in the light-emittinglayer was examined with the use of Light-emitting Element 2-1. First,the angle dependence of the shape of the EL emission spectrum wasmeasured by measuring the EL spectrum in steps of 1° in such a mannerthat, as shown in FIG. 20, the substrate provided with Light-emittingElement 2-1 was inclined to a detector (a photonic multichannel analyzerPMA-12, produced by Hamamatsu Photonics K.K.) from θ=0° to 80°. In thismeasurement, a linear polarizer (Glan-Taylor prism) was disposed betweenLight-emitting Element 2-1 and the detector to be perpendicular to thesubstrate surface in order to remove an S polarization from lightemitted from Light-emitting Element 2-1, so that the spectrum of only aP polarization was measured.

FIG. 25 is a graph in which the vertical axis represents the integratedintensity of the EL emission spectrum from 440 nm to 956 nm depending onthe angle (θ) and the horizontal axis represents the angle (θ) of thedetector. In FIG. 25, a curve plotted by open squares represents themeasured values, and a solid curve and a dashed curve represent thecalculation results obtained by setfos which is an organic devicesimulator. The calculation was performed by inputting the thickness ofeach layer in the element, the measured values of the refractive indexand the extinction efficiency, the measured value of the emissionspectrum of a dopant, the position and the width of a light-emittingregion, and the orientation parameter a. Among them, the thickness ofeach layer, the refractive index, and the extinction efficiency weremeasured with the use of a spectroscopic ellipsometer (M-2000U, producedby J.A. Woollam Japan Corp.). For the measurement, a film was used whichwas formed of the light-emitting material over a quartz substrate by avacuum evaporation method to a thickness of 150 nm. The emissionspectrum of the dopant was measured using a fluorescencespectrophotometer (FS920, produced by Hamamatsu Photonics K.K.). For themeasurement, a film was used which was formed by co-evaporation of2mDBTBPDBq-II, PCBBiF, and Ir(mpmppm)₂(acac) using a vacuum evaporationmethod at a weight ratio of 0.8:0.2:0.05(=2mDBTBPDBq-II:PCBBiF:Ir(mpmppm)₂(acac)) over a quartz substrate to athickness of 50 nm. In the calculation by setfos, further, thelight-emitting region was set. The light-emitting region was assumed tospread such that the recombination probability was attenuatedexponentially in the cathode direction with the interface between thehole-transport layer and the light-emitting layer as a top,specifically, the recombination probability was attenuated to 1/e at athickness of 25 nm. Thus, the angle dependence of the integratedintensity of the emission spectrum corresponding to each parameter a canbe calculated In Light-emitting Element 2-1, the measured value wellfitted a curve of a=0.18.

FIG. 26 is a 2D contour map obtained by measuring the angle dependenceof the EL emission spectrum of Light-emitting Element 2-1. FIG. 27 is a2D contour map obtained by calculation. FIG. 26 and FIG. 27 show thatthese 2D contour maps match well. This indicates that the orientation ofthe light-emitting materials in Light-emitting Elements 2 and 2-1 wasaccurately obtained in both the experiment and the calculation.

The parameter a is ⅓≈0.33 in the case where the transition dipole isoriented in a random direction, and the parameter a is 0 in the casewhere the transition dipole is completely parallel to the substrate. Thelight extraction efficiency in the case of a=0 is 1.5 times the lightextraction efficiency in the case of a=⅓≈0.33. In view of this, thelight-emitting element of this example in which a=0.18 has lightextraction efficiency that is 1.23 times the light extraction efficiencyof the light-emitting element with a random orientation.

Light-emitting Elements 2 and 2-1 are formed using the same material fortheir light-emitting layers by the same method; thus, it can be saidthat Light-emitting Element 2 has, like Light-emitting Element 2-1, anorientation of a=0.18. Light-emitting Element 2 has a very high externalquantum efficiency of 30%. It is found that a light-emitting elementwith high emission efficiency can be obtained by setting a to less thanor equal to 0.2. The quantum yield of the film was 0.84 on average inthe case of excitation light at 370 nm; the film was formed byco-evaporation of 2mDBTBPDBq-II, PCBBiF, and Ir(mpmppm)₂(acac) using avacuum evaporation method at a weight ratio of 0.8:0.2:0.05(=2mDBTBPDBq-II:PCBBiF:Ir(mpmppm)₂(acac)) to a thickness of 50 nm. Thatis, the light-emitting layer of this example has a parameter a of 0.2 orless and a phosphorescent quantum yield of 0.84 or higher; thus, thislayer satisfies theoretical conditions under which the external quantumefficiency can be higher than or equal to 30%.

Example 3

In this example, results of calculating the parameter a of alight-emitting element (Light-emitting Element 3) of one embodiment ofthe present invention with high efficiency are described in detail. Forthe above purpose, a light-emitting element (Light-emitting Element 3-1)for measurement which includes a light-emitting layer having the samestructure as that of Light-emitting Element 3 and in which the luminancein the front direction is reduced as much as possible was alsofabricated.

First, a fabrication method and the structure of the light-emittingelement of one embodiment of the present invention are described.Organic compounds used in the light-emitting element of one embodimentof the present invention are given below.

(Fabrication Method of Light-Emitting Element 3)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness of the first electrode 101 was70 nm and the electrode area was 2 mm×2 mm.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation to a thickness of 75 nm at a weightratio of 2:1 (=DBT3P-II: molybdenum oxide) by an evaporation methodusing resistance heating, so that the hole-injection layer 111 wasformed.

Next, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:BPAFLP) represented by Structural Formula (ii) was deposited byevaporation to a thickness of 20 nm on the hole-injection layer 111 toform the hole-transport layer 112.

Then, the light-emitting layer 113 was formed in the following manner:2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (vii),N-(1,1′-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9H-fluoren-2-amine (abbreviation: PCBBiF) represented by Structural Formula(viii), andbis{2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-1N]-4,6-dimethylphenyl-κC}(2,2,6,6-tetramethyl-3,5-heptanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-dmp)₂(dpm)]) represented by Structural Formula(x) were deposited by co-evaporation at a weight ratio of 0.8:0.2:0.05(=2mDBTBPDBq-II:PCBBiF:[Ir(dmdppr-dmp)₂(dpm)]) to a thickness of 40 nm.

Then, on the light-emitting layer 113, 2mDBTBPDBq-II was deposited byevaporation to a thickness of 30 nm, and bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (v) wasdeposited by evaporation to a thickness of 15 nm to form theelectron-transport layer 114.

After the formation of the electron-transport layer 114, lithium oxide(Li₂O) was deposited by evaporation to a thickness of 0.1 nm to form theelectron-injection layer 115. Then, aluminum was deposited byevaporation to a thickness of 200 nm to form the second electrode 102.Through the above-described steps, Light-emitting Element 3 of thisexample was fabricated.

(Fabrication Method of Light-Emitting Element 3-1)

Light-emitting element 3-1 was fabricated in the following manner. Afterthe electron-injection layer 115 of Light-emitting Element 3 was formed,copper phthalocyanine (abbreviation: CuPc) represented by StructuralFormula (vi) was deposited by evaporation to a thickness of 2 nm, andthen, DBT3P-II and molybdenum(VI) oxide were deposited by co-evaporationat a weight ratio of 2:1 (=DBT3P-II: molybdenum oxide) to a thickness of100 nm. In such a manner, a layer for adjusting the thickness wasformed.

The element structures of Light-emitting Elements 3 and 3-1 are shown inthe following table.

TABLE 5 hole-injection hole-transport light-emitting electron-injectionlayer for adjusting layer layer layer electron-transport layer layer thethickness 75 nm 20 nm 40 nm 30 nm 15 nm 0.1 nm 2 nm 90 nm Element 3DBT3P-II:MoOx BPAFLP 2mDBTBPDBq-II: 2mDBTBPDBq-II BPhen Li₂O — — (4:2)PCBBiF: Element 3-1 [Ir(dmdppr-dmp)₂(dpm)] CuPc DBT3P-II:MoOx(0.8:0.2:0.05) (4:2)

Light-emitting elements 3 and 3-1 were each sealed using a glasssubstrate in a glove box containing a nitrogen atmosphere so as not tobe exposed to the air (specifically, a sealant was applied to surroundthe element and UV treatment and heat treatment at 80° C. for 1 hourwere performed at the time of sealing). Then, the initialcharacteristics of Light-emitting elements 3 and 3-1 were measured. Notethat the measurement was carried out in an atmosphere kept at 25° C.

FIG. 28 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 3. Table 6 shows the main characteristics ofLight-emitting Elements 3 and 3-1 at a luminance of about 1000 cd/m².

TABLE 6 External Current Current Quantum Voltage Current densitychromaticity Efficiency Efficiency (V) (mA) (mA/cm²) x y (cd/A) (%)Element 3 3.2 0.10 2.5 0.67 0.33 38 27 Element 3-1 5.0 1.77 44 0.62 0.351.2 2.0

As shown in Table 6, Light-emitting Element 3 has a very highefficiency, e.g., an external quantum efficiency of 27%. Thelight-emitting material used in Light-emitting Element 3,[Ir(dmdppr-dmp)₂(dpm)], has an emission quantum yield (ϕ) of 0.79.Assuming that the carrier balance (γ) is 1 and the proportion ofgenerated excitons (α) is 1, the light extraction efficiency (χ) iscalculated to 34.1%, which is much higher than a general theoreticallight extraction efficiency of 20% to 30%.

Light-emitting Element 3-1 includes the layer for adjusting thethickness in addition to the components of Light-emitting Element 3. Byadjusting the optical path length, light that travels in the frontdirection is attenuated, so that a representing the orientation statecan be easily obtained. A difference between the structure and thefabrication method of Light-emitting Element 3 and those ofLight-emitting Element 3-1 lies only in the layer for adjusting thethickness; thus, the orientation states of light-emitting substances inthe light-emitting layers are assumed to be the same.

The orientation state of a light-emitting material in the light-emittinglayer was examined with the use of Light-emitting Element 3-1. Themethod is the same as those in Examples 1 and 2 and its description isthus omitted.

FIG. 29 is a graph in which the vertical axis represents the integratedintensity of the EL emission spectrum from 570 nm to 900 nm depending onthe angle (θ) and the horizontal axis represents the angle (θ) of thedetector. In FIG. 29, a curve plotted by open squares represents themeasured values, and a solid curve and a dashed curve represent thecalculation results obtained by setfos which is an organic devicesimulator. The calculation was performed by inputting the thickness ofeach layer in the element, the measured values of the refractive indexand the extinction efficiency, the measured value of the emissionspectrum of a dopant, the position and the width of a light-emittingregion, and the orientation parameter a. Among them, the thickness ofeach layer, the refractive index, and the extinction efficiency weremeasured with the use of a spectroscopic ellipsometer (M-2000U, producedby J.A. Woollam Japan Corp.). For the measurement, a film was used whichwas formed of the light-emitting material over a quartz substrate by avacuum evaporation method to a thickness of 150 nm. The emissionspectrum of the dopant was measured using a fluorescencespectrophotometer (FS920, produced by Hamamatsu Photonics K.K.). For themeasurement, a film was used which was formed by co-evaporation of2mDBTBPDBq-II, PCBBiF, and [Ir(dmdppr-dmp)₂(dpm)] using a vacuumevaporation method at a weight ratio of 0.8:0.2:0.05(=2mDBTBPDBq-II:PCBBiF:[Ir(dmdppr-dmp)₂(dpm)]) over a quartz substrateto a thickness of 50 nm. In the calculation by setfos, further, thelight-emitting region was set. The light-emitting region was assumed tospread such that the recombination probability was attenuated inaccordance with the Gaussian distribution with a point which isapproximately 38 nm from the interface between the hole-transport layerand the light-emitting layer as a top, specifically, the distancebetween inflection points of an assumed Gaussian function was 28 nm.Thus, the angle dependence of the integrated intensity of the emissionspectrum corresponding to each parameter a can be calculated. InLight-emitting Element 3-1, the measured value well fitted a curve ofa=0.16.

FIG. 30 is a 2D contour map obtained by measuring the angle dependenceof the EL emission spectrum of Light-emitting Element 3-1. FIG. 31 is a2D contour map obtained by calculation. FIG. 30 and FIG. 31 show thatthese 2D contour maps match well. This indicates that the orientation ofthe light-emitting materials in Light-emitting Elements 3 and 3-1 wasaccurately obtained in both the experiment and the calculation.

The parameter a is ⅓≈0.33 in the case where the transition dipole isoriented in a random direction, and the parameter a is 0 in the casewhere the transition dipole is completely parallel to the substrate. Thelight extraction efficiency in the case of a=0 is 1.5 times the lightextraction efficiency in the case of a=⅓≈0.33. In view of this, thelight-emitting element of this example in which a=0.16 has lightextraction efficiency that is 1.26 times the light extraction efficiencyof the light-emitting element with a random orientation.

Light-emitting Elements 3 and 3-1 are formed using the same material fortheir light-emitting layers by the same method; thus, it can be saidthat, thanks to [Ir(dmdppr-dmp)₂(dpm)] in the light-emitting layer,Light-emitting Element 3 has, like Light-emitting Element 3-1, anorientation of a=0.16. Light-emitting Element 3 has a very high externalquantum efficiency of 27%. It is found that a light-emitting elementwith high emission efficiency can be obtained by setting a to less thanor equal to 0.2.

Example 4

In this example, results of calculating the parameter a of alight-emitting element (Light-emitting Element 4) of one embodiment ofthe present invention with high efficiency are described in detail. Forthe above purpose, a light-emitting element (Light-emitting Element 4-1)for measurement which includes a light-emitting layer having the samestructure as that of Light-emitting Element 4 and in which the luminancein the front direction is reduced as much as possible was alsofabricated.

First, a fabrication method and the structure of the light-emittingelement of one embodiment of the present invention are described.Organic compounds used in the light-emitting element of one embodimentof the present invention are given below.

(Fabrication Method of Light-Emitting Element 4)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness of the first electrode 101 was70 nm and the electrode area was 2 mm×2 mm.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation to a thickness of 20 nm at a weightratio of 2:1 (=DBT3P-II: molybdenum oxide) by an evaporation methodusing resistance heating, so that the hole-injection layer 111 wasformed.

Next, 3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP) representedby Structural Formula (xi) was deposited by evaporation to a thicknessof 20 nm on the hole-injection layer 111 to form the hole-transportlayer 112.

Then, the light-emitting layer 113 was formed in the following manner:PCCP, 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation:4,6mCzP2Pm) represented by Structural Formula (xii), andtris{2-[4-(4-cyano-2,6-diisobutylphenyl)-5-(2-methylphenyl)-4H-1,2,4-triazol-3-yl-κN²]phenyl-κC}iridium(III)(abbreviation: [Ir(mpptz-diBuCNp)₃]) represented by Structural Formula(xiii) were deposited by co-evaporation to a thickness of 30 nm at aweight ratio of 0.6:0.4:0.125 (=PCCP: 4,6mCzP2Pm: [Ir(mpptz-diBuCNp)₃]),and then, PCCP, 4,6mCzP2Pm, and [Ir(mpptz-diBuCNp)₃] were deposited byco-evaporation to a thickness of 10 nm at a weight ratio of0.2:0.8:0.125 (=PCCP: 4,6mCzP2Pm: [Ir(mpptz-diBuCNp)₃]).

Then, on the light-emitting layer 113, 4,6mCzP2Pm was deposited byevaporation to a thickness of 10 nm, and bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (v) wasdeposited by evaporation to a thickness of 15 nm to form theelectron-transport layer 114.

After the formation of the electron-transport layer 114, lithium oxide(Li₂O) was deposited by evaporation to a thickness of 0.1 nm to form theelectron-injection layer 115. Then, aluminum was deposited byevaporation to a thickness of 200 nm to form the second electrode 102.Through the above-described steps, Light-emitting Element 4 of thisexample was fabricated.

(Fabrication Method of Light-Emitting Element 4-1)

Light-emitting element 4-1 was fabricated in the following manner. Afterthe electron-injection layer 115 of Light-emitting Element 4 was formed,copper phthalocyanine (abbreviation: CuPc) represented by StructuralFormula (vi) was deposited by evaporation to a thickness of 2 nm, andthen, DBT3P-II and molybdenum(VI) oxide were deposited by co-evaporationat a weight ratio of 2:1 (=DBT3P-II: molybdenum oxide) to a thickness of55 nm. In such a manner, a layer for adjusting the thickness was formed.

The element structures of Light-emitting Elements 4 and 4-1 are shown inthe following table.

TABLE 7 hole-injection hole-transport light-emitting electron-injectionlayer for adjusting layer layer layer electron-transport layer layer thethickness 20 nm 20 nm 30 nm 10 nm 10 nm 15 nm 0.1 nm 2 nm 55 nm Element4 DBT3P-II:MoOx PCCP *1 *2 4,6mCzP2Pm BPhen Li₂O — — (4:2) Element 4-1CuPc DBT3P-II:MoOx (4:2) *1 PCCP:4,6mCzP2Pm:[Ir(mpptz-diBuCNp)₃](0.6:0.4:0.125) *2 PCCP:4,6mCzP2Pm:[Ir(mpptz-diBuCNp)₃] (0.2:0.8:0.125)

Light-emitting Elements 4 and 4-1 were each sealed using a glasssubstrate in a glove box containing a nitrogen atmosphere so as not tobe exposed to the air (specifically, a sealant was applied to surroundthe element and UV treatment and heat treatment at 80° C. for 1 hourwere performed at the time of sealing). Then, the initialcharacteristics of Light-emitting Elements 4 and 4-1 were measured. Notethat the measurement was carried out in an atmosphere kept at 25° C.

FIG. 32 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 4. Table 8 shows the main characteristics ofLight-emitting Elements 4 and 4-1 at a luminance of about 1000 cd/m².

TABLE 8 External Current Current Quantum Voltage Current densitychromaticity Efficiency Efficiency (V) (mA) (mA/cm²) x y (cd/A) (%)Element 4 3.2 0.03 0.7 0.21 0.53 84 30 Element 4-1 4.4 0.52 13 0.33 0.457.7 3.0

As shown in Table 8, Light-emitting Element 4 has a very highefficiency, e.g., an external quantum efficiency of 30%. Thelight-emitting material used in Light-emitting Element 4,[Ir(mpptz-diBuCNp)₃], has an emission quantum yield (ϕ) of 0.93.Assuming that the carrier balance (γ) is 1 and the proportion ofgenerated excitons (α) is 1, the light extraction efficiency (χ) iscalculated to 32.3%, which is much higher than a general theoreticallight extraction efficiency of 20% to 30%.

Light-emitting Element 4-1 includes the layer for adjusting thethickness in addition to the components of Light-emitting Element 4. Byadjusting the optical path length, light that travels in the frontdirection is attenuated, so that a representing the orientation statecan be easily obtained. A difference between the structure and thefabrication method of Light-emitting Element 4 and those ofLight-emitting Element 4-1 lies only in the layer for adjusting thethickness; thus, the orientation states of light-emitting substances inthe light-emitting layers are assumed to be the same.

The orientation state of a light-emitting material in the light-emittinglayer was examined with the use of Light-emitting Element 4-1. Themethod is the same as those in Examples 1 and 2 and its description isthus omitted.

FIG. 33 is a graph in which the vertical axis represents the integratedintensity of the EL emission spectrum from 350 nm to 810 nm depending onthe angle (θ) and the horizontal axis represents the angle (θ) of thedetector. In FIG. 33, a curve plotted by open squares represents themeasured values, and a solid curve and a dashed curve represent thecalculation results obtained by setfos which is an organic devicesimulator. The calculation was performed by inputting the thickness ofeach layer in the element, the measured values of the refractive indexand the extinction efficiency, the measured value of the emissionspectrum of a dopant, the position and the width of a light-emittingregion, and the orientation parameter a. Among them, the thickness ofeach layer, the refractive index, and the extinction efficiency weremeasured with the use of a spectroscopic ellipsometer (M-2000U, producedby J.A. Woollam Japan Corp.). For the measurement, a film was used whichwas formed of the light-emitting material over a quartz substrate by avacuum evaporation method to a thickness of 150 nm. The emissionspectrum of the dopant was measured using a fluorescencespectrophotometer (FS920, produced by Hamamatsu Photonics K.K.). For themeasurement, a film was used which was formed by co-evaporation of PCCP,4,6mCzP2Pm, and [Ir(mpptz-diBuCNp)₃] at a weight ratio of 0.6:0.4:0.125(=PCCP: 4,6mCzP2Pm: Ir(mpptz-diBuCNp)₃) over a quartz substrate to athickness of 15 nm. In the calculation by setfos, further, thelight-emitting region was set. The light-emitting region was assumed tospread such that the recombination probability was attenuated inaccordance with the Gaussian distribution with a point which isapproximately 20 nm from the interface between the hole-transport layerand the light-emitting layer as a top, specifically, the distancebetween inflection points of an assumed Gaussian function was 30 nm.Thus, the angle dependence of the integrated intensity of the emissionspectrum corresponding to each parameter a can be calculated. InLight-emitting Element 4-1, the measured value well fitted a curve ofa=0.15.

FIG. 34 is a 2D contour map obtained by measuring the angle dependenceof the EL emission spectrum of Light-emitting Element 4-1. FIG. 35 is a2D contour map obtained by calculation. FIG. 34 and FIG. 35 show thatthese 2D contour maps match well. This indicates that the orientation ofthe light-emitting materials in Light-emitting Elements 4 and 4-1 wasaccurately obtained in both the experiment and the calculation.

The parameter a is ⅓≈0.33 in the case where the transition dipole isoriented in a random direction, and the parameter a is 0 in the casewhere the transition dipole is completely parallel to the substrate. Thelight extraction efficiency in the case of a=0 is 1.5 times the lightextraction efficiency in the case of a=⅓≈0.33. In view of this, thelight-emitting element of this example in which a=0.15 has lightextraction efficiency that is 1.28 times the light extraction efficiencyof the light-emitting element with a random orientation.

Light-emitting Elements 4 and 4-1 are formed using the same material fortheir light-emitting layers by the same method; thus, it can be saidthat, thanks to [Ir(mpptz-diBuCNp)₃] in the light-emitting layer,Light-emitting Element 4 has, like Light-emitting Element 4-1, anorientation of a=0.15. Light-emitting Element 4 has a very high externalquantum efficiency of 30%. It is found that a light-emitting elementwith high emission efficiency can be obtained by setting a to less thanor equal to 0.2.

The quantum yield of the film was 80% on average in the case ofexcitation light at 350 nm; the film was formed by co-evaporation ofPCCP, 4,6mCzP2Pm, and [Ir(mpptz-diBuCNp)₃] at a weight ratio of0.6:0.4:0.125 (=PCCP: 4,6mCzP2Pm: [Ir(mpptz-diBuCNp)₃]) to a thicknessof 50 nm. That is, the light-emitting layer of this example has aparameter a of 0.2 or less and a phosphorescent quantum yield of 80% orhigher; thus, this layer satisfies theoretical conditions under whichthe external quantum efficiency can be higher than or equal to 30%.

Example 5

In this example, results of calculating the parameter a of alight-emitting element (Light-emitting Element 5) of one embodiment ofthe present invention with high efficiency are described in detail. Forthe above purpose, a light-emitting element (Light-emitting Element 5-1)for measurement which includes a light-emitting layer having the samestructure as that of Light-emitting Element 5 and in which the luminancein the front direction is reduced as much as possible was alsofabricated.

First, a fabrication method and the structure of the light-emittingelement of one embodiment of the present invention are described.Organic compounds used in the light-emitting element of one embodimentof the present invention are given below.

(Fabrication Method of Light-Emitting Element 5)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness of the first electrode 101 was70 nm and the electrode area was 2 mm×2 mm.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation to a thickness of 20 nm at a weightratio of 2:1 (=DBT3P-II: molybdenum oxide) by an evaporation methodusing resistance heating, so that the hole-injection layer 111 wasformed.

Next, 4,4′-bis(9-carbazole)-2,2′-dimethylbiphenyl (abbreviation: dmCBP)represented by Structural Formula (xiv) was deposited by evaporation toa thickness of 20 nm on the hole-injection layer 111 to form thehole-transport layer 112.

Then, 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:35DCzPPy) represented by Structural Formula (xv) andtris{2-[4-(2-adamantyl)-3-methyl-4H-1,2,4-triazol-5-yl-N]phenyl-κC}iridium(III)(abbreviation: [Ir(Mptz-Adm2)₃]) represented by Structural Formula (xvi)were deposited by co-evaporation to a thickness of 30 nm at a weightratio of 1:0.06 (=35DCzPPy: [Ir(Mptz-Adm2)₃]), whereby thelight-emitting layer 113 was formed.

After that, on the light-emitting layer 113,1,3,5-tris[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB)represented by Structural Formula (xvii) was deposited by evaporation toa thickness of 25 nm to form the electron-transport layer 114.

After the formation of the electron-transport layer 114, lithium oxide(Li₂O) was deposited by evaporation to a thickness of 0.1 nm to form theelectron-injection layer 115. Then, aluminum was deposited byevaporation to a thickness of 200 nm to form the second electrode 102.Through the above-described steps, Light-emitting Element 5 of thisexample was fabricated.

(Fabrication Method of Light-Emitting Element 5-1)

Light-emitting element 5-1 was fabricated in the following manner. Afterthe electron-injection layer 115 of Light-emitting Element 5 was formed,copper phthalocyanine (abbreviation: CuPc) represented by StructuralFormula (vi) was deposited by evaporation to a thickness of 2 nm, andthen, DBT3P-II and molybdenum(VI) oxide were deposited by co-evaporationat a weight ratio of 2:1 (=DBT3P-II: molybdenum oxide) to a thickness of60 nm. In such a manner, a layer for adjusting the thickness was formed.

The element structures of Light-emitting Elements 5 and 5-1 are shown inthe following table.

TABLE 9 hole-injection hole-transport light-emitting electron-transportelectron-injection layer for adjusting layer layer layer layer layer thethickness 20 nm 20 nm 30 nm 25 nm 0.1 nm 2 nm 60 nm Element 5DBT3P-II:MoOx dmCBP 35DCzPPy: TmPyPB Li₂O — — (4:2) [Ir(Mptz-Adm)₃]Element 5-1 (1:0.06) CuPc DBT3P-II:MoOx (4:2)

Light-emitting elements 5 and 5-1 were each sealed using a glasssubstrate in a glove box containing a nitrogen atmosphere so as not tobe exposed to the air (specifically, a sealant was applied to surroundthe element and UV treatment and heat treatment at 80° C. for 1 hourwere performed at the time of sealing). Then, the initialcharacteristics of Light-emitting Elements 5 and 5-1 were measured. Notethat the measurement was carried out in an atmosphere kept at 25° C.

FIG. 36 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 5. Table 10 shows the main characteristics ofLight-emitting Elements 5 and 5-1 at a luminance of about 1000 cd/m².

TABLE 10 External Current Current Quantum Voltage Current densitychromaticity Efficiency Efficiency (V) (mA) (mA/cm²) x y (cd/A) (%)Element 5 3.9 0.10 2.5 0.15 0.19 42 25 Element 5-1 6.2 0.68 17 0.32 0.486.1 2.0

As shown in Table 10, Light-emitting Element 5 has a very highefficiency, e.g., an external quantum efficiency of 25%. Thelight-emitting material used in Light-emitting Element 5,[Ir(Mptz-Adm2)₃], has an emission quantum yield (ϕ) of 0.94. Assumingthat the carrier balance (γ) is 1 and the proportion of generatedexcitons (α) is 1, the light extraction efficiency (χ) is calculated to26.6%.

Light-emitting Element 5-1 includes the layer for adjusting thethickness in addition to the components of Light-emitting Element 5. Byadjusting the optical path length, light that travels in the frontdirection is attenuated, so that a representing the orientation statecan be easily obtained. A difference between the structure and thefabrication method of Light-emitting Element 5 and those ofLight-emitting Element 5-1 lies only in the layer for adjusting thethickness; thus, the orientation states of light-emitting substances inthe light-emitting layers are assumed to be the same.

The orientation state of a light-emitting material in the light-emittinglayer was examined with the use of Light-emitting Element 5-1. First,the angle dependence of the shape of the EL emission spectrum wasmeasured by measuring the EL spectrum in steps of 10° in such a mannerthat, as shown in FIG. 20, the substrate provided with Light-emittingElement 5-1 was inclined to a detector (a photonic multichannel analyzerPMA-12, produced by Hamamatsu Photonics K.K.) from θ=0° to 80°. In thismeasurement, a linear polarizer (Glan-Taylor prism) was disposed betweenLight-emitting Element 5-1 and the detector to be perpendicular to thesubstrate surface in order to remove an S polarization from lightemitted from Light-emitting Element 5-1, so that the spectrum of only aP polarization was measured.

FIG. 37 is a graph in which the vertical axis represents the integratedintensity of the EL emission spectrum from 400 nm to 800 nm depending onthe angle (θ) and the horizontal axis represents the angle (θ) of thedetector. In FIG. 37, a curve plotted by open squares represents themeasured values, and a solid curve and a dashed curve represent thecalculation results obtained by setfos which is an organic devicesimulator. The calculation was performed by inputting the thickness ofeach layer in the element, the measured values of the refractive indexand the extinction efficiency, the measured value of the emissionspectrum of a dopant, the position and the width of a light-emittingregion, and the orientation parameter a. Among them, the thickness ofeach layer, the refractive index, and the extinction efficiency weremeasured with the use of a spectroscopic ellipsometer (M-2000U, producedby J.A. Woollam Japan Corp.). For the measurement, a film was used whichwas formed of the light-emitting material over a quartz substrate by avacuum evaporation method to a thickness of 150 nm. The emissionspectrum of the dopant was measured using a fluorescencespectrophotometer (FS920, produced by Hamamatsu Photonics K.K.). For themeasurement, a film was used which was formed by co-evaporation of35DCzPPy and [Ir(Mptz-Adm2)₃] using a vacuum evaporation method at aweight ratio of 1:0.06 (=35DCzPPy: [Ir(Mptz-Adm2)₃]) over a quartzsubstrate to a thickness of 50 nm. In the calculation by setfos,further, the light-emitting region was set. The light-emitting regionwas assumed to spread such that the recombination probability wasattenuated exponentially in the cathode direction with the interfacebetween the hole-transport layer and the light-emitting layer as a top,specifically, the recombination probability was attenuated to 1/e at athickness of 5 nm. Thus, the angle dependence of the integratedintensity of the emission spectrum corresponding to each parameter a canbe calculated. In Light-emitting Element 5-1, the measured value wellfitted a curve of a=0.16.

The parameter a is ⅓≈0.33 in the case where the transition dipole isoriented in a random direction, and the parameter a is 0 in the casewhere the transition dipole is completely parallel to the substrate. Thelight extraction efficiency in the case of a=0 is 1.5 times the lightextraction efficiency in the case of a=⅓≈0.33. In view of this, thelight-emitting element of this example in which a=0.16 has lightextraction efficiency that is 1.26 times the light extraction efficiencyof the light-emitting element with a random orientation.

Light-emitting Elements 5 and 5-1 are formed using the same material fortheir light-emitting layers by the same method; thus, it can be saidthat, thanks to [Ir(Mptz-Adm2)₃] in the light-emitting layer,Light-emitting Element 5 has, like Light-emitting Element 5-1, anorientation of a=0.16. Light-emitting Element 5 has a very high externalquantum efficiency of 25%. It is found that a light-emitting elementwith high emission efficiency can be obtained by setting a to less thanor equal to 0.2.

Example 6

In this example, results of calculating the parameter a of alight-emitting element (Light-emitting Element 6) of one embodiment ofthe present invention with high efficiency are described in detail. Forthe above purpose, a light-emitting element (Light-emitting Element 6-1)for measurement which includes a light-emitting layer having the samestructure as that of Light-emitting Element 6 and in which the luminancein the front direction is reduced as much as possible was alsofabricated. FIG. 18 illustrates the structure of the light-emittingelement.

First, a fabrication method and the structure of the light-emittingelement of one embodiment of the present invention are described.Organic compounds used in the light-emitting element of one embodimentof the present invention are given below.

(Fabrication Method of Light-Emitting Element 6)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness of the first electrode 101 was70 nm and the electrode area was 2 mm×2 mm.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation to a thickness of 40 nm at a weightratio of 4:2 (=DBT3P-II: molybdenum oxide) by an evaporation methodusing resistance heating, so that the hole-injection layer 111 wasformed.

Next, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:BPAFLP) represented by Structural Formula (ii) was deposited byevaporation to a thickness of 30 nm on the hole-injection layer 111 toform the hole-transport layer 112.

Next, the light-emitting layer 113 was formed by co-evaporation of7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) represented by Structural Formula (iii) andN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-anthracene-9,10-diamine(abbreviation: 9,10mMemFLPA2A) represented by Structural Formula (xviii)at a weight ratio of 1:0.1 (=cgDBCzPA: 9,10mMemFLPA2A) to a thickness of35 nm.

Then, on the light-emitting layer 113, bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (v) wasdeposited by evaporation to a thickness of 15 nm to form theelectron-transport layer 114.

After the formation of the electron-transport layer 114, lithium oxide(Li₂O) was deposited by evaporation to a thickness of 0.1 nm to form theelectron-injection layer 115. Then, aluminum was deposited byevaporation to a thickness of 200 nm to form the second electrode 102.Through the above-described steps, Light-emitting Element 6 of thisexample was fabricated.

(Fabrication Method of Light-Emitting Element 6-1)

Light-emitting element 6-1 was fabricated in the following manner. Afterthe electron-injection layer 115 of Light-emitting Element 6 was formed,copper phthalocyanine (abbreviation: CuPc) represented by StructuralFormula (vi) was deposited by evaporation to a thickness of 2 nm to formthe electron-relay layer 118, and then, DBT3P-II and molybdenum(VI)oxide were deposited by co-evaporation at a weight ratio of 2:1(=DBT3P-II: molybdenum oxide) to a thickness of 80 nm to form the p-typelayer 117. In such a manner, a layer for adjusting the thickness wasformed.

The element structures of Light-emitting Elements 6 and 6-1 are shown inthe following table.

TABLE 11 hole-injection hole-transport light-emitting electron-transportelectron-injection layer for adjusting layer layer layer layer layer thethickness 40 nm 30 nm 35 nm 15 nm 0.1 nm 2 nm 80 nm Element 6DBT3P-II:MoOx BPAFLP cgDBCzPA: BPhen Li₂O — — (4:2) 9,10mMemFLPA2AElement 6-1 (1:0.1) CuPc DBT3P-II:MoOx (4:2)

Light-emitting Elements 6 and 6-1 were each sealed using a glasssubstrate in a glove box containing a nitrogen atmosphere so as not tobe exposed to the air (specifically, a sealant was applied to surroundthe element and UV treatment and heat treatment at 80° C. for 1 hourwere performed at the time of sealing). Then, the initialcharacteristics of Light-emitting Elements 6 and 6-1 were measured. Notethat the measurement was carried out in an atmosphere kept at 25° C.

FIG. 38 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 6. Table 12 shows the main characteristics ofLight-emitting Elements 6 and 6-1 at a luminance of about 1000 cd/m².

TABLE 12 External Current Current Quantum Voltage Current densitychromaticity Efficiency Efficiency (V) (mA) (mA/cm²) x y (cd/A) (%)Element 6 3.0 0.09 2.4 0.29 0.65 34 8.3 Element 6-1 4.8 3.40 85 0.330.46 1.2 0.5

As shown in Table 12, Light-emitting Element 6 has a very highefficiency, e.g., an external quantum efficiency of 8.3%. Thelight-emitting material used in Light-emitting Element 6,9,10mMemFLPA2A, has an emission quantum yield (ϕ) of 0.90. Assuming thatthe carrier balance (γ) is 1 and the proportion of generated excitons(α) is 0.25, the light extraction efficiency (χ) is calculated to 36.9%,which is much higher than a general theoretical light extractionefficiency of 20% to 30%.

Light-emitting Element 6-1 includes the layer for adjusting thethickness in addition to the components of Light-emitting Element 6. Byadjusting the optical path length, light that travels in the frontdirection is attenuated, so that a representing the orientation statecan be easily obtained. A difference between the structure and thefabrication method of Light-emitting Element 6 and those ofLight-emitting Element 6-1 lies only in the layer for adjusting thethickness; thus, the orientation states of light-emitting substances inthe light-emitting layers are assumed to be the same.

The orientation state of a light-emitting material in the light-emittinglayer was examined with the use of Light-emitting Element 6-1. First,the angle dependence of the shape of the EL emission spectrum wasmeasured by measuring the EL spectrum in steps of 1° in such a mannerthat, as shown in FIG. 20, the substrate provided with Light-emittingElement 6-1 was inclined to a detector (a photonic multichannel analyzerPMA-12, produced by Hamamatsu Photonics K.K.) from θ=0° to 80°. In thismeasurement, a linear polarizer (Glan-Taylor prism) was disposed betweenLight-emitting Element 6-1 and the detector to be perpendicular to thesubstrate surface in order to remove an S polarization from lightemitted from Light-emitting Element 6-1, so that the spectrum of only aP polarization was measured.

FIG. 39 is a graph in which the vertical axis represents the integratedintensity of the EL emission spectrum from 470 nm to 750 nm depending onthe angle (θ) and the horizontal axis represents the angle (θ) of thedetector. In FIG. 39, a curve plotted by open squares represents themeasured values, and a solid curve and a dashed curve represent thecalculation results obtained by setfos which is an organic devicesimulator. The calculation was performed by inputting the thickness ofeach layer in the element, the measured values of the refractive indexand the extinction efficiency, the measured value of the emissionspectrum of a dopant, the position and the width of a light-emittingregion, and the orientation parameter a. Among them, the thickness ofeach layer, the refractive index, and the extinction efficiency weremeasured with the use of a spectroscopic ellipsometer (M-2000U, producedby J.A. Woollam Japan Corp.). For the measurement, a film was used whichwas formed of the light-emitting material over a quartz substrate by avacuum evaporation method to a thickness of 150 nm. The emissionspectrum of the dopant was measured using a fluorescencespectrophotometer (FS920, produced by Hamamatsu Photonics K.K.). For themeasurement, a film was used which was formed by co-evaporation ofcgDBCzPA and 9,10mMemFLPA2A using a vacuum evaporation method at aweight ratio of 1:0.1 (=cgDBCzPA: 9,10mMemFLPA2A) over a quartzsubstrate to a thickness of 50 nm. In the calculation by setfos,further, the light-emitting region was set. The light-emitting regionwas assumed to spread such that the recombination probability wasattenuated in accordance with the Gaussian distribution with a pointwhich is approximately 33 nm from the interface between thehole-transport layer and the light-emitting layer as a top,specifically, the distance between inflection points of an assumedGaussian function was 12.5 nm. Thus, the angle dependence of theintegrated intensity of the emission spectrum corresponding to eachparameter a can be calculated. In Light-emitting Element 6-1, themeasured value well fitted a curve of a=0.19.

FIG. 40 is a 2D contour map obtained by measuring the angle dependenceof the EL emission spectrum of Light-emitting Element 6-1. FIG. 41 is a2D contour map obtained by calculation. FIG. 40 and FIG. 41 show thatthese 2D contour maps match well. This indicates that the orientation ofthe light-emitting materials in Light-emitting Elements 6 and 6-1 wasaccurately obtained in both the experiment and the calculation.

The parameter a is ⅓≈0.33 in the case where the transition dipole isoriented in a random direction, and the parameter a is 0 in the casewhere the transition dipole is completely parallel to the substrate. Thelight extraction efficiency in the case of a=0 is 1.5 times the lightextraction efficiency in the case of a=⅓≈0.33. In view of this, thelight-emitting element of this example in which a=0.19 has lightextraction efficiency that is 1.22 times the light extraction efficiencyof the light-emitting element with a random orientation. That is, thelight-emitting element of one embodiment of the present invention has anemission efficiency that is 1.22 times the emission efficiency of thelight-emitting element with a random orientation.

Light-emitting Elements 6 and 6-1 are formed using the same material fortheir light-emitting layers by the same method; thus, it can be saidthat Light-emitting Element 6 has, like Light-emitting Element 6-1, anorientation of a=0.19. Light-emitting Element 6 has a very high externalquantum efficiency of 8.3%. It is found that a light-emitting elementwith high emission efficiency can be obtained by setting a to less thanor equal to 0.2.

Example 7

In this example, results of calculating the parameter a of alight-emitting element (Light-emitting Element 7) of one embodiment ofthe present invention with high efficiency are described in detail. Forthe above purpose, a light-emitting element (Light-emitting Element 7-1)for measurement which includes a light-emitting layer having the samestructure as that of Light-emitting Element 7 and in which the luminancein the front direction is reduced as much as possible was alsofabricated. FIG. 18 illustrates the structure of the light-emittingelement.

First, a fabrication method and the structure of the light-emittingelement of one embodiment of the present invention are described.Organic compounds used in the light-emitting element of one embodimentof the present invention are given below.

(Fabrication Method of Light-Emitting Element 7)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness of the first electrode 101 was70 nm and the electrode area was 2 mm×2 mm.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. After that,2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT-CN) represented by Structural Formula (xix) was deposited byevaporation to a thickness of 10 nm on the first electrode 101 by anevaporation method using resistance heating, whereby the hole-injectionlayer 111 was formed.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)represented by Structural Formula (xx) was deposited by evaporation to athickness of 80 nm on the hole-injection layer 111 to form thehole-transport layer 112.

Next, the light-emitting layer 113 was formed by co-evaporation ofrubrene represented by Structural Formula (xxi),9-(1-naphthyl)-10-(2-naphthyl)anthracene (abbreviation: α,β-ADN)represented by Structural Formula (xxii), and5,10,15,20-tetraphenylbisbenzo[5,6]indeno[1,2,3-cd:1′,2′,3′-lm]perylene(abbreviation: DBP) represented by Structural Formula (xxiii) at aweight ratio of 0.8:0.2:0.005 (=rubrene: α,β-ADN: DBP) to a thickness of30 nm.

Then, on the light-emitting layer 113, bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (v) wasdeposited by evaporation to a thickness of 20 nm to form theelectron-transport layer 114.

After the formation of the electron-transport layer 114, lithium oxide(Li₂O) was deposited by evaporation to a thickness of 0.1 nm to form theelectron-injection layer 115. Then, aluminum was deposited byevaporation to a thickness of 200 nm to form the second electrode 102.Through the above-described steps, Light-emitting Element 7 of thisexample was fabricated.

(Fabrication Method of Light-Emitting Element 7-1)

Light-emitting element 7-1 was fabricated in the following manner. Afterthe electron-injection layer 115 of Light-emitting Element 7 was formed,copper phthalocyanine (abbreviation: CuPc) represented by StructuralFormula (vi) was deposited by evaporation to a thickness of 2 nm to formthe electron-relay layer 118, and then, DBT3P-II and molybdenum(VI)oxide were deposited by co-evaporation at a weight ratio of 2:1(=DBT3P-II: molybdenum oxide) to a thickness of 85 nm to form the p-typelayer 117. In such a manner, a layer for adjusting the thickness wasformed.

The element structures of Light-emitting Elements 7 and 7-1 are shown inthe following table.

TABLE 13 hole-injection hole-transport light-emitting electron-injectionlayer for adjusting layer layer layer electron-transport layer layer thethickness 10 nm 80 nm 30 nm 30 nm 20 nm 0.1 nm 2 nm 85 nm Element 7HAT-CN NPB Rubrene: α,β-ADN BPhen Li₂O — — α,β-ADN:DBP Element 7-1(0.8:0.2:0.005) CuPc DBT3P-II:MoOx (4:2)

Light-emitting Elements 7 and 7-1 were each sealed using a glasssubstrate in a glove box containing a nitrogen atmosphere so as not tobe exposed to the air (specifically, a sealant was applied to surroundthe element and UV treatment and heat treatment at 80° C. for 1 hourwere performed at the time of sealing). Then, the initialcharacteristics of Light-emitting Elements 7 and 7-1 were measured. Notethat the measurement was carried out in an atmosphere kept at 25° C.

FIG. 42 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 7. Table 14 shows the main characteristics ofLight-emitting Elements 7 and 7-1 at a luminance of about 1000 cd/m².

TABLE 14 External Current Current Quantum Voltage Current densitychromaticity Efficiency Efficiency (V) (mA) (mA/cm²) x y (cd/A) (%)Element 7 3.2 0.32 8.0 0.68 0.32 14 11 Element 7-1 6.6 15.06 377 0.670.33 0.2 0.7

As shown in Table 14, Light-emitting Element 7 has a very highefficiency, e.g., an external quantum efficiency of 11%. Thelight-emitting material used in Light-emitting Element 7, DBP, has anemission quantum yield (ϕ) of 0.72. Assuming that the carrier balance(γ) is 1 and the proportion of generated excitons (α) is 0.25, the lightextraction efficiency (χ) is calculated to 61.1%, which is much higherthan a general theoretical light extraction efficiency of 20% to 30%.Measurement results of transient EL emission indicate that TTA alsooccurred in this element. The proportion of generated excitons waspractically higher than 0.25 owing to TTA.

Light-emitting Element 7-1 includes the layer for adjusting thethickness in addition to the components of Light-emitting Element 7. Byadjusting the optical path length, light that travels in the frontdirection is attenuated, so that a representing the orientation statecan be easily obtained. A difference between the structure and thefabrication method of Light-emitting Element 7 and those ofLight-emitting Element 7-1 lies only in the layer for adjusting thethickness; thus, the orientation states of light-emitting substances inthe light-emitting layers are assumed to be the same.

The orientation state of a light-emitting material in the light-emittinglayer was examined with the use of Light-emitting Element 7-1. First,the angle dependence of the shape of the EL emission spectrum wasmeasured by measuring the EL spectrum in steps of 1° in such a mannerthat, as shown in FIG. 20, the substrate provided with Light-emittingElement 7-1 was inclined to a detector (a photonic multichannel analyzerPMA-12, produced by Hamamatsu Photonics K.K.) from θ=0° to 80°. In thismeasurement, a linear polarizer (Glan-Taylor prism) was disposed betweenLight-emitting Element 7-1 and the detector to be perpendicular to thesubstrate surface in order to remove an S polarization from lightemitted from Light-emitting Element 7-1, so that the spectrum of only aP polarization was measured.

FIG. 43 is a graph in which the vertical axis represents the integratedintensity of the EL emission spectrum from 570 nm to 900 nm depending onthe angle (θ) and the horizontal axis represents the angle (θ) of thedetector. In FIG. 43, a curve plotted by open squares represents themeasured values, and a solid curve and a dashed curve represent thecalculation results obtained by setfos which is an organic devicesimulator. The calculation was performed by inputting the thickness ofeach layer in the element, the measured values of the refractive indexand the extinction efficiency, the measured value of the emissionspectrum of a dopant, the position and the width of a light-emittingregion, and the orientation parameter a. Among them, the thickness ofeach layer, the refractive index, and the extinction efficiency weremeasured with the use of a spectroscopic ellipsometer (M-2000U, producedby J.A. Woollam Japan Corp.). For the measurement, a film was used whichwas formed of the light-emitting material over a quartz substrate by avacuum evaporation method to a thickness of 150 nm. The emissionspectrum of the dopant was measured using a fluorescencespectrophotometer (FS920, produced by Hamamatsu Photonics K.K.). For themeasurement, a film was used which was formed by co-evaporation ofrubrene, α,β-ADN, and DBP using a vacuum evaporation method at a weightratio of 0.8:0.2:0.005 (=rubrene: α,β-ADN: DBP) over a quartz substrateto a thickness of 50 nm. In the calculation by setfos, further, thelight-emitting region was set. The light-emitting region was assumed tospread such that the recombination probability was attenuated inaccordance with the Gaussian distribution with a point which isapproximately 6 nm from the interface between the hole-transport layerand the light-emitting layer as a top, specifically, the distancebetween inflection points of an assumed Gaussian function was 25 nm.Thus, the angle dependence of the integrated intensity of the emissionspectrum corresponding to each parameter a can be calculated. InLight-emitting Element 7-1, the measured value well fitted a curve ofa=0.12.

FIG. 44 is a 2D contour map obtained by measuring the angle dependenceof the EL emission spectrum of Light-emitting Element 7-1. FIG. 45 is a2D contour map obtained by calculation. FIG. 44 and FIG. 45 show thatthese 2D contour maps match well. This indicates that the orientation ofthe light-emitting materials in Light-emitting Elements 7 and 7-1 wasaccurately obtained in both the experiment and the calculation.

The parameter a is ⅓≈0.33 in the case where the transition dipole isoriented in a random direction, and the parameter a is 0 in the casewhere the transition dipole is completely parallel to the substrate. Thelight extraction efficiency in the case of a=0 is 1.5 times the lightextraction efficiency in the case of a=⅓≈0.33. In view of this, thelight-emitting element of this example in which a=0.12 has lightextraction efficiency that is 1.32 times the light extraction efficiencyof the light-emitting element with a random orientation. That is, thelight-emitting element of one embodiment of the present invention has anemission efficiency that is 1.32 times the emission efficiency of thelight-emitting element with a random orientation.

Light-emitting Elements 7 and 7-1 are formed using the same material fortheir light-emitting layers by the same method; thus, it can be saidthat Light-emitting Element 7 has, like Light-emitting Element 7-1, anorientation of a=0.12. Light-emitting Element 7 has a very high externalquantum efficiency of 11%. It is found that a light-emitting elementwith high emission efficiency can be obtained by setting a to less orequal to 0.2. Measurement results of transient EL emission indicate thatTTA also occurred in this element.

Example 8

In this example, results of calculating the parameter a of alight-emitting element (Light-emitting Element 8) of one embodiment ofthe present invention with high efficiency are described in detail. Forthe above purpose, a light-emitting element (Light-emitting Element 8-1)for measurement which includes a light-emitting layer having the samestructure as that of Light-emitting Element 8 and in which the luminancein the front direction is reduced as much as possible was alsofabricated.

First, a fabrication method and the structure of the light-emittingelement of one embodiment of the present invention are described.Organic compounds used in the light-emitting element of one embodimentof the present invention are given below.

(Fabrication Method of Light-Emitting Element 8)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness of the first electrode 101 was70 nm and the electrode area was 2 mm×2 mm.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation to a thickness of 50 nm at a weightratio of 4:2 (=DBT3P-II: molybdenum oxide) by an evaporation methodusing resistance heating, so that the hole-injection layer 111 wasformed.

Then, 9-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-9H-carbazole (abbreviation:mCzFLP) represented by Structural Formula (xxiv) was deposited byevaporation to a thickness of 20 nm on the hole-injection layer 111 toform the hole-transport layer 112.

Then, the light-emitting layer 113 was formed in the following manner:4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm)represented by Structural Formula (xii) andbis[2-(6-tert-butyl-4-pyrimidinyl-κN³)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]) represented by Structural Formula(xxv) were deposited by co-evaporation at a weight ratio of 1:0.05(=4,6mCzP2Pm: [Ir(tBuppm)₂(acac)]) to a thickness of 40 nm.

Then, on the light-emitting layer 113, 4,6mCzP2Pm was deposited byevaporation to a thickness of 15 nm, and bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (v) wasdeposited by evaporation to a thickness of 10 nm to form theelectron-transport layer 114.

After the formation of the electron-transport layer 114, lithium oxide(Li₂O) was deposited by evaporation to a thickness of 0.1 nm to form theelectron-injection layer 115. Then, aluminum was deposited byevaporation to a thickness of 200 nm to form the second electrode 102.Through the above-described steps, Light-emitting Element 8 of thisexample was fabricated.

(Fabrication Method of Light-Emitting Element 8-1)

Light-emitting element 8-1 was fabricated in the following manner. Afterthe electron-injection layer 115 of Light-emitting Element 8 was formed,copper phthalocyanine (abbreviation: CuPc) represented by StructuralFormula (vi) was deposited by evaporation to a thickness of 2 nm, andthen, DBT3P-II and molybdenum(VI) oxide were deposited by co-evaporationat a weight ratio of 2:1 (=DBT3P-II: molybdenum oxide) to a thickness of80 nm. In such a manner, a layer for adjusting the thickness was formed.

The element structures of Light-emitting Elements 8 and 8-1 are shown inthe following table.

TABLE 15 hole-injection hole-transport light-emitting electron-injectionlayer for adjusting layer layer layer electron-transport layer layer thethickness 50 nm 20 nm 40 nm 15 nm 10 nm 0.1 nm 2 nm 80 nm Element 8DBT3P-II:MoOx mCzFLP 4,6mCzP2Pm: 4,6CzP2Pm BPhen Li₂O — — (4:2)[Ir(tBuppm)₂(acac)] Element 8-1 (1:0.05) CuPc DBT3P-II:MoOx (4:2)

Light-emitting Elements 8 and 8-1 were each sealed using a glasssubstrate in a glove box containing a nitrogen atmosphere so as not tobe exposed to the air (specifically, a sealant was applied to surroundthe element and UV treatment and heat treatment at 80° C. for 1 hourwere performed at the time of sealing). Then, the initialcharacteristics of Light-emitting Elements 8 and 8-1 were measured. Notethat the measurement was carried out in an atmosphere kept at 25° C.

FIG. 46 shows the external quantum efficiency-luminance characteristicsof Light-emitting Element 8. Table 16 shows the main characteristics ofLight-emitting Elements 8 and 8-1 at a luminance of about 1000 cd/m².

TABLE 16 External Current Current Quantum Voltage Current densitychromaticity Efficiency Efficiency (V) (mA) (mA/cm²) x y (cd/A) (%)Element 8 3.4 0.03 0.8 0.40 0.59 128 31 Element 8-1 5.6 1.06 27 0.430.53 3.7 1.7

As shown in Table 16, Light-emitting Element 8 has a very highefficiency, e.g., an external quantum efficiency of 31%. Thelight-emitting material used in Light-emitting Element 8,[Ir(tBuppm)₂(acac)], has an emission quantum yield (ϕ) of 0.91. Assumingthat the carrier balance (γ) is 1 and the proportion of generatedexcitons (α) is 1, the light extraction efficiency (χ) is calculated to34.1%, which is much higher than a general theoretical light extractionefficiency of 20% to 30%.

Light-emitting Element 8-1 includes the layer for adjusting thethickness in addition to the components of Light-emitting Element 8. Byadjusting the optical path length, light that travels in the frontdirection is attenuated, so that a representing the orientation statecan be easily obtained. A difference between the structure and thefabrication method of Light-emitting Element 8 and those ofLight-emitting Element 8-1 lies only in the layer for adjusting thethickness; thus, the orientation states of light-emitting substances inthe light-emitting layers are assumed to be the same.

The orientation state of a light-emitting material in the light-emittinglayer was examined with the use of Light-emitting Element 8-1. First,the angle dependence of the shape of the EL emission spectrum wasmeasured by measuring the EL spectrum in steps of 1° in such a mannerthat, as shown in FIG. 20, the substrate provided with Light-emittingElement 8-1 was inclined to a detector (a photonic multichannel analyzerPMA-12, produced by Hamamatsu Photonics K.K.) from θ=0° to 80°. In thismeasurement, a linear polarizer (Glan-Taylor prism) was disposed betweenLight-emitting Element 8-1 and the detector to be perpendicular to thesubstrate surface in order to remove an S polarization from lightemitted from Light-emitting Element 8-1, so that the spectrum of only aP polarization was measured.

FIG. 47 is a graph in which the vertical axis represents the integratedintensity of the EL emission spectrum from 480 nm to 800 nm depending onthe angle (θ) and the horizontal axis represents the angle (θ) of thedetector. In FIG. 47, a curve plotted by open squares represents themeasured values, and a solid curve and a dashed curve represent thecalculation results obtained by setfos which is an organic devicesimulator. The calculation was performed by inputting the thickness ofeach layer in the element, the measured values of the refractive indexand the extinction efficiency, the measured value of the emissionspectrum of a dopant, the position and the width of a light-emittingregion, and the orientation parameter a. Among them, the thickness ofeach layer, the refractive index, and the extinction efficiency weremeasured with the use of a spectroscopic ellipsometer (M-2000U, producedby J.A. Woollam Japan Corp.). For the measurement, a film was used whichwas formed of the light-emitting material over a quartz substrate by avacuum evaporation method to a thickness of 150 nm. The emissionspectrum of the dopant was measured using a fluorescencespectrophotometer (FS920, produced by Hamamatsu Photonics K.K.). For themeasurement, a film was used which was formed by co-evaporation of4,6mCzP2Pm and [Ir(tBuppm)₂(acac)] using a vacuum evaporation method ata weight ratio of 1:0.05 (=4,6mCzP2Pm: [Ir(tBuppm)₂(acac)]) over aquartz substrate to a thickness of 40 nm. In the calculation by setfos,further, the light-emitting region was set. The light-emitting regionwas assumed to spread such that the recombination probability wasattenuated in accordance with the Gaussian distribution with theinterface between the hole-transport layer and the light-emitting layeras a top, specifically, the distance between inflection points of anassumed Gaussian function was 11.5 nm. Thus, the angle dependence of theintegrated intensity of the emission spectrum corresponding to eachparameter a can be calculated. In Light-emitting Element 8-1, themeasured value well fitted a curve of a=0.19.

FIG. 48 is a 2D contour map obtained by measuring the angle dependenceof the EL emission spectrum of Light-emitting Element 8-1. FIG. 49 is a2D contour map obtained by calculation. FIG. 48 and FIG. 49 show thatthese 2D contour maps match well. This indicates that the orientation ofthe light-emitting materials in Light-emitting Elements 8 and 8-1 wasaccurately obtained in both the experiment and the calculation.

The parameter a is ⅓≈0.33 in the case where the transition dipole isoriented in a random direction, and the parameter a is 0 in the casewhere the transition dipole is completely parallel to the substrate. Thelight extraction efficiency in the case of a=0 is 1.5 times the lightextraction efficiency in the case of a=⅓≈0.33. In view of this, thelight-emitting element of this example in which a=0.19 has lightextraction efficiency that is 1.22 times the light extraction efficiencyof the light-emitting element with a random orientation.

Light-emitting Elements 8 and 8-1 are formed using the same material fortheir light-emitting layers by the same method; thus, it can be saidthat Light-emitting Element 8 has, like Light-emitting Element 8-1, anorientation of a=0.19. Light-emitting Element 8 has a very high externalquantum efficiency of 31%. It is found that a light-emitting elementwith high emission efficiency can be obtained by setting a to less thanor equal to 0.2.

This application is based on Japanese Patent Application Serial No.2016-101789 filed with Japan Patent Office on May 20, 2016, and JapanesePatent Application Serial No. 2016-122964 filed with Japan Patent Officeon Jun. 21, 2016, the entire contents of which are hereby incorporatedby reference.

What is claimed is:
 1. A light-emitting element comprising: a firstelectrode; an EL layer over the first electrode, the EL layercomprising: a light-emitting layer comprising a first substance and asecond substance; and a second electrode over the EL layer, wherein anamount of the first substance is larger than an amount of the secondsubstance in the light-emitting layer, wherein each molecule of thesecond substance in the light-emitting layer has an average transitiondipole moment, wherein the second substance in the light-emitting layerhas molecular orientation parameter a, wherein the molecular orientationparameter a is a proportion of TMv component in the whole averagetransition dipole moments, wherein the TMv component is a vectorcomponent in a direction perpendicular to the first electrode among thewhole average transition dipole moments, and wherein the molecularorientation parameter a is greater than 0 and less than or equal to 0.2.2. The light-emitting element according to claim 1, wherein molecules ofthe second substance in the light-emitting layer are oriented.
 3. Thelight-emitting element according to claim 1, wherein the light-emittinglayer is deposited by a vacuum evaporation method under an atmospherewhich has a higher volume ratio of a partial pressure of carbon dioxideto the total pressure than the air.
 4. The light-emitting elementaccording to claim 1, wherein the light-emitting layer is deposited by avacuum evaporation method under an atmosphere which has a firstpercentage of a partial pressure of carbon dioxide with respect to thetotal pressure, and wherein the first percentage is 0.1% or higher and10% or less.
 5. The light-emitting element according to claim 1, whereinthe light-emitting layer further comprises a third substance, andwherein the first substance and the third substance is configured toform an exciplex.
 6. The light-emitting element according to claim 1,wherein the second substance is a fluorescent substance.
 7. Thelight-emitting element according to claim 1, wherein the secondsubstance has a condensed aromatic hydrocarbon skeleton.
 8. Thelight-emitting element according to claim 6, wherein an external quantumefficiency of the light-emitting element is higher than or equal to 10%.9. The light-emitting element according to claim 1, wherein lightemission from the light-emitting element comprises a delayed fluorescentcomponent.
 10. The light-emitting element according to claim 1, whereinthe molecular orientation parameter a is greater than 0 and less than orequal to 0.15.
 11. A light-emitting device comprising: thelight-emitting element according to claim 1; and at least one of atransistor and a substrate.
 12. An electronic device comprising: thelight-emitting device according to claim 11; and at least one of asensor, an operation button, a speaker and a microphone.
 13. A lightingdevice comprising: the light-emitting device according to claim 11; anda housing.
 14. A light-emitting element comprising: a first electrode;an EL layer over the first electrode, the EL layer comprising: alight-emitting layer comprising a first substance and a secondsubstance; and a second electrode over the EL layer, wherein an amountof the first substance is larger than an amount of the second substancein the light-emitting layer, wherein the second substance is an iridiumcomplex, wherein each molecule of the second substance in thelight-emitting layer has an average transition dipole moment, whereinthe second substance in the light-emitting layer has molecularorientation parameter a, wherein the molecular orientation parameter ais a proportion of TMv component in the whole average transition dipolemoments, wherein the TMv component is a vector component in a directionperpendicular to the first electrode among the whole average transitiondipole moments, and wherein the molecular orientation parameter a isgreater than 0 and less than or equal to 0.2.
 15. The light-emittingelement according to claim 14, wherein the iridium complex comprises anazole skeleton.
 16. The light-emitting element according to claim 15,wherein the azole skeleton is a triazole skeleton.
 17. Thelight-emitting element according to claim 14, wherein the light-emittinglayer further comprises a third substance, and wherein the firstsubstance and the third substance is configured to form an exciplex. 18.The light-emitting element according to claim 14, wherein thelight-emitting layer is deposited by a vacuum evaporation method underan atmosphere which has a higher volume ratio of a partial pressure ofcarbon dioxide to the total pressure than the air.
 19. Thelight-emitting element according to claim 14, wherein the light-emittinglayer is deposited by a vacuum evaporation method under an atmospherewhich has a first percentage of a partial pressure of carbon dioxidewith respect to the total pressure, and wherein the first percentage is0.1% or higher and 10% or less.
 20. The light-emitting element accordingto claim 14, wherein the molecular orientation parameter a is greaterthan 0 and less than or equal to 0.15.